My husband is doing his student teaching this semester, and he's got a kindergarten class right now too! He would prefer third or fourth grade, but you know how it is. Anyway we've been thinking about whether he will put a tank in his room, when/if he ever gets hired...
I asked this question on another forum the other day, asking for suggestions... I learned a lot!
First things first, check with the administration to be sure you're allowed to have a tank. If so, you should probably mention to the maintenance staff that you will be setting up a tank in your class, so they can give you any guidelines they have, and they can also help you keep an eye on it after hours.
You can do guppies, just do all males. Voila, no babies!
If you do shrimp, you could do ghost shrimp, 'cause they look cool, are super cheap, and seem to be pretty much indestructable.
And, if you want to go super-SUPER-easy, what about snails? Apple snails come in all kind of cool colors, and then of course nerites will keep the glass clean for you! Kids love to watch the nerites scrape the glass with their cute little mouths!
Just a few suggestions, hope they help! Good luck and have fun!!!