The puffer(s) would be cool, but that's a very different concept than a community tank.
So the tank is going to be in the classroom? Is it going to have to be near an outside window? I'm just thinking that you might get a decent amount of algae in there after awhile, so 3-4 ottos would probably be a good idea.
Are you interested in plants?
There are a lot of totally different things you could go with, so you kinda have to pick:
1) puffer tank
2) a planted tank with a school of something
3) just a general community tank
If you want a school, I'd do something like:
1 dwarf gourami
7 rasboras or 5 Hyphessobrycon tetras (Lemon/Flame/Phantom/Serpae/Red)
3 ottos
1 mystery snail
Then I'd see what plants you can get ahold of easily and make it pretty heavily planted. The gourami should be aggressive enough to keep the school together pretty well. You could also put a female betta in there or roll the dice with a male betta.
If you just want a general community tank, endlers or dwarf platys are both pretty cool. Around here the LFS have dwarf platys but not endlers.
Seems like any tank can fit some ottos and some bottom feeders no matter what's in there. The problem with a 10 gal. is that you can't really fit 5-6 corys too well, and most ppl say they'll be stressed if you only have 1-3. I'd say there's probably no problem putting 4 panda corys in a 10 gal , and they're really cute. You also might be interested in having shrimp instead of corys for the bottom.
Something else cool to do would be a tank with a crayfish, but that might be distracting/upsetting for students at times