A small group of lamprologus multifasciatus (tanganyikan shell dweller). Interesting behavior, easy to spawn, fascinating to watch them move sand and shells.
a single oscar would be neat as you could buy small and the kids could see him grow large although at his full grown size will need to be rehomed maybe. The pair of convicts would also be cool for a science class because you could touch on the basics of genetics I think we did punit squares in 6th grade. Shell dwellers are also a really cute and fun fish for a grade school tank.