If you are only getting 8-12 members, and you are doing a Swiss, you should be able to secure a clear winner in 3 rounds but if you get up to 16 you need 4 rounds.
The number of rounds and total time available, determine the maximum time control. So, 4 hours without major breaks, means each round should be done in an hour at most. For 4 rounds,G/30 would be pushing it, especially if any rounds go to the full time control.
G/25 would give you 50 minutes per round, max, leaving 10 minutes between rounds to handle small breaks and pairings. Of course, if you want to use delay or increment, that will change things some. If you decide to do that take the base + delay to determine the time needed. G/25,d5 is essentially the same as G/30.
I plan to run my first chess tournament in about a month. Most tournaments here for newbies (and I am honestly about 1 step above that) use a static 30-minute clock. Whoever has reached checkmate or has the most material on the board is declared the winner. For total beginners it's fine, but my players are a little more skilled now and know how to use clocks. The thing I'm struggling with is how to set the time control. I know that the tournament will be one-day only, and will probably be confined to around 4 hours or less (small school). I'll get more time if I'm lucky, but I can't count on it. I will run SwissSys for pairings, but I'm not sure which time controls to use. How long is appropriate per side? At least 15 minutes, right? FYI, I expect to get about 8-12 students to show up. Certainly not more than 16.