Sea monkeys/ brine shrimp were great. I remember taking them home and setting them on a desk, returning a few days later to find them all dead.
Hatching/ attempting to keep brine shrimp, at least, in a small container, proved to be very smelly and futile... therefore, I am not sure how well it would work as a class display... thing.
In-class displays (not projects, which i realize is what you were asking for

): I would suggest anoles (not to be handled). All you need is a 10 gallon terrarium or so and plenty of small crickets. Also, gerbils, which in my borther's 4th grade were taken home by one student every weekend (seemed to be relatively stressful, and they died before the end of the school year). Guppy fry sounded like a good idea to me.
Project/ Worms: in 6th grade we got meal worms and raised them until they became beetles. That's a possible idea, methinks.
Unfortunately, that's all that comes to mind, though this thread seems to have been forgotten, in which case I would also suggest you go into the general suggestion/ hobbies and try there.
HTH