get some corydoras, they snuffle round on the bottom and are always great to watch.
eber tertas, neon tetras, harliquins, a few more mollys?
Kihuli loachs are another good one for a class room, my friends were always interested in mine.
most fish that can put up with a little bullying and don't look or have anything that looks like food or another male sould be fine.
You could even get some suggestions and get your class to choose.
Man if I ever become a teacher I'm putting a tank in my class =)