I haven't checked your games but my guess is you fail to realize what's going on at every corners of the board at once, and that's why your students keep telling you that you need see the board.
you probably miss a bishop positioned on a far diagonal, or a rook defending a piece from miles away.
if that's the case, just stop before every move and double check your corners 
even though how your next move seems pretty obvious and smart to you.
We can play some unrated 10 min chess if you want some tailored advice afterwards on anything I observe.
Also, you shouldn't be playing blitz when starting out and 3 minutes is fast blitz at that! It is usually better to stick to 10 min games or longer for learning (preferably even longer if you can afford to). Let me briefly answer your questions though:
1) Openings are overrated at beginner and intermediate level. If you do study opening(s), then yes it is usually better to stick with just one and learn that well, but it is more about understanding the ideas than memorizing moves. In fact, I recommend just sticking to opening principles instead of learning a particular opening until you are at least 1000 rating (with some exceptions like if you've been playing for a long time already). https://www.chess.com/blog/KeSetoKaiba/opening-principles-again
2) There is no singular checklist to keep in mind because chess is a complex game which layers ideas and some things apply more often only in certain situations (and even then with exceptions!).
However, there are some basic guidelines which tend to be true in most all situations and when starting out, here is a good list to get started with. You can layer more concepts to remember once you internalize these:
- Before moving: do a "blunder check" which means to visually make sure nothing is hanging or that there are no one-move threats by the opponent if you make that move.
- When calculating, usually look at checks, captures and forcing moves first. These are good to start with because if something works here, the opponent has less replies to choose from and this means easier for you to calculate.
- Learn basic checkmates (like King + Queen vs King or King + Rook vs King) and basic theoretical endgames (like King + pawn vs King) as these patterns are extremely important to chess and can help you win many games.
I started learning chess this year, at a similar age.
You might gain a lot, especially at shorter time controls, to pick a system for each color, and run with it. Get familiar kinds of setups every game, get comfortable with them. Get yourself in familiar territory and make your opponent have to think.
If you want checklists? Look at videos by GothamChess on basic principles (Checks, Captures, Attacks), and the first 5-10 videos of a "speed run" series by Daniel Naroditsky, in which he has talked about making natural, principled opening moves without having to know any specific "theory."
If your rating is stuck at 900 then yes you do need to see the board better, your games (just based on your rating, I haven't looked) are probably dominated by blunders both in the form of hanging pieces and missed opportunities. So if you can work on this you'll get better results and I would certainly prioritise that over specific opening study.
But reading between the lines it sounds like maybe you're putting too much emphasis on the improvement side of things and while improving is nice, it sounds like you've already taken the most important step with connecting with your struggling students, which is to even play chess at all, and 900 level is not horrendous in the grand scheme of things.
Learning just one opening will improve your short-term results, by giving you a bit of a "home ground" advantage in the opening of most games.
But it will slow down your rate of improvement to some extent, by limiting the variety of different middle-game positions (and plans) that you are exposed to.
Why do people who play mainly rapid and blitz complain when they have trouble improving? Play 1 day move games so you have time to study your position and actually think about what you are doing.
Learning not to hang pieces is so much more important than learning some fancy opening.
It's a bit hard to imagine that the struggling kids are playing chess.
In the USA I'd expect them to be up the back of the class with gun magazines open on their screens, all the while feeling convinced that they're unique precious petals.
First, stop playing anything slower than 15 min Rapid. You are new to the game, you need time to think. Fast games are just going to overwhelm and make you feel like you can't play (guess what, you're new, you can't play that fast, you don't have the information ingrained at that level ... yet - like your students take longer to recall information that you can bring up immediately, it takes time to ingrain those patterns ).
As for "should you learn one opening"? Well, you will get lots of advice, mostly around "just learn opening principles" (which, by the way, is what I was going to say). But really, that's not helpful, is it. So, my advice, for what it is worth, is learn 3 openings. As White, play the Italian. Simply, e4, Nf3, and Bc4, you can play it against the Sicilian even (it's not a good way to play against the Sicilian, but Magnus has done it, and won, so it isn't losing necessarily either - but what Magnus does doesn't really count for us mortals). Still, it is solid, it conforms to opening principles, and most likely none of your opponents are going to know all that many moves. So pick a line - any line - and go for it. Adapt based upon what your opponents tend to play. But those first 3 moves will, generally, not get you into too much trouble.
Now, I said 3 openings. So against 1. e4, play 1. ... e5. And 90% of the time you will get 2. Nf3 so play 2 Nf6, and learn Petrov's Défense. Good, solid, counter attack on the centre, and it will serve you until you become World Champion. Against 1. d4, play the Dutch (1. ... f4). Very imbalanced, will teach you different skills, but also you can play it against the London, or King's Indian, etc. By combining Petrov's and the Dutch as your "Black game", you have solid dependable, and also sharp tactical, so defence and offense training. By just playing the Italian (for awhile), you have good solid set up as White, focus on the pressuring the weak Black f7 pawn, and you eventually can expand to things like Evan's Gambit, Scotch Gambit, and various lines of the Italian (Guicco Piano, Moeller attack, Mas Lang Attack, Bird's Attack, etc) depending upon what suits you.
The opening is just a way to place your pieces. Chess begins when the opening is done, provided both have done their homework. If you Play 1. e4 and find you get the French a lot, then learn one line against the French. Same for other openings. For the Sicilian, 1. e4 c4 go for the Grand Prix Attack (2. Nc3 .... 3 f4 ... with ideas of Bb5 or Bc4). That will last you until a rating of around 2000 - 2200, by which time you should have learned other lines anyway. For now, you just want to have a few simple ways of getting to "something familiar". These will do that. For the French, for example, if you get that a lot, try the advance (1. e4 e6 2. d4 d5 3. e4 ...), it's not the best line, but it's solid, and White can pose some problems which are easy to understand what you're doing.
In the end, chess is a game of patterns. But you have to learn the patterns, so find a way to force the same patterns as often as you can, and then, and only then, will you learn from them.
@marcosmas03 There is great advice above, but I will add the following:
If your rating is stuck at 900 then yes you do need to see the board better, your games (just based on your rating, I haven't looked) are probably dominated by blunders both in the form of hanging pieces and missed opportunities. So if you can work on this you'll get better results and I would certainly prioritise that over specific opening study.
But reading between the lines it sounds like maybe you're putting too much emphasis on the improvement side of things and while improving is nice, it sounds like you've already taken the most important step with connecting with your struggling students, which is to even play chess at all, and 900 level is not horrendous in the grand scheme of things.
Thank you for the kind words. In the grand scheme of things connecting with my students is the most important thing. Plus, they really do enjoy destroying me.
It's a bit hard to imagine that the struggling kids are playing chess.
In the USA I'd expect them to be up the back of the class with gun magazines open on their screens, all the while feeling convinced that they're unique precious petals.
Schools in the US are not that bad. The students who are struggling are the ones who think they are so smart that they do not need to do the work. So I made deals with them, that if they beat me at chess and score above an A on the test, I will exempt a daily assignment. It's a win win for everyone. They listen more to me during class. They have done better on the tests then previously. More importantly, it gives us something other than school to talk about.
We can play some unrated 10 min chess if you want some tailored advice afterwards on anything I observe.
Also, you shouldn't be playing blitz when starting out and 3 minutes is fast blitz at that! It is usually better to stick to 10 min games or longer for learning (preferably even longer if you can afford to). Let me briefly answer your questions though:
1) Openings are overrated at beginner and intermediate level. If you do study opening(s), then yes it is usually better to stick with just one and learn that well, but it is more about understanding the ideas than memorizing moves. In fact, I recommend just sticking to opening principles instead of learning a particular opening until you are at least 1000 rating (with some exceptions like if you've been playing for a long time already). https://www.chess.com/blog/KeSetoKaiba/opening-principles-again
2) There is no singular checklist to keep in mind because chess is a complex game which layers ideas and some things apply more often only in certain situations (and even then with exceptions!).
However, there are some basic guidelines which tend to be true in most all situations and when starting out, here is a good list to get started with. You can layer more concepts to remember once you internalize these:
- Before moving: do a "blunder check" which means to visually make sure nothing is hanging or that there are no one-move threats by the opponent if you make that move.
- When calculating, usually look at checks, captures and forcing moves first. These are good to start with because if something works here, the opponent has less replies to choose from and this means easier for you to calculate.
- Learn basic checkmates (like King + Queen vs King or King + Rook vs King) and basic theoretical endgames (like King + pawn vs King) as these patterns are extremely important to chess and can help you win many games.
Thank you so much for this wonderful advice.
Already very good advice on here but I'll contribute 1. NO blitz!! 2. basic endings 3. basic opening principles 4. tactics 5. PAWN STRUCTURES. Studying pawn structures will guide you in planning.
Yeah... I used to be half-decent at the game.
Then I got old.
... and my brain turned to porridge...
It seems to be the common fate.
@marcosmas03 -
You may discover something helpful in these...
Chess Courses - Instructional Resources...
https://www.chess.com/blog/RussBell/beginners-chess-course-instructional-resources
Improving Your Chess - Resources for Beginners and Beyond...
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
https://www.chess.com/blog/RussBell
I teach history at a public school and started to play chess to connect with students who were struggling. My problem is that I am not improving at all. I have watched videos, I have had students try to teach me, (if one more student says that I just need to see the board, I will scream), and I have tried lots of the lessons and tips on this site. I have been stuck at 900 on Rapid and 1500 on puzzles, and am a whopping 400 in 3 minutes. So fellow chess fans I ask you these questions: 1. Should I focus on learning one opening, if so which one? 2. Is there a simple checklist that I need to get tattooed on my hand in order to start getting some wins?
Hope everyone is doing well, and thank you in advance for any comments and suggestions.