I have to disagree with the african suggestion. For one, you will need to spend a lot of money just to stock the tank with fish. Yes they are colorful and pretty but not overly personable. Kids seem more impressed with a single large colorful fish then a bunch of little fish. At least that has been my experience. Depending upon what size tank you get I would go with either oscars or midevils. Both are cheap, colorfull, get large, have great personality, eat anything, and have no special water requirements.
Teachers are good at finding 'resources'.