Actually, Bunnies and Guinea Pigs should only be kept indoors, IMHO. But I don't think they are at all suitable for a class full of young kids, they can be very shy with too much activity, and they would be left alone for most of the day.

I would think, either 2+ rats (that go home with a teacher for weekends/summer) or fish. I suggest a nice planted 5-10 gallon tank on a stand or on the teacher's desk with 1 pretty male Betta and maybe a couple of African dwarf frogs would be perfect. Bettas can happily go weekends without eating, and it wouldn't be a huge mess to take him home over the summer. They also live 3-4 years when treated well, and when acquired from a reputable aquarium shop or breeder.