Ok, first of all, what are you kids like, typically?
I work at a private boarding school with lots of learning disabilities....especially ADD and ADHD.
I wanted some sort of pet in the classroom when I started the job. I opted for a pair of rats. Bad idea really. Way too active and they just disrupted the class.
Lesson learned: Don't get something too active 
Then I went for an albino horned frog. Kids didn't care for that either. Heck, it pretty much just stays buried so they never see it. I have to admit they like the feedings however. It also isn't handleable.
Lesson learned: Don't get something too inactive 
In short, find something that fits all critera for you AND the kids.
Then last year I was given a mexican black kingsnake. Decent sized beast at over 4 feet, but very tame and handleable (don't forget to have a supply of alcohol-based antibacterial hand sanitizer on hand...they rarely remember to wash up after handling). He is also active enough that the kids see him all the time, but he's not too active that he makes alot of noise or distracts the kids.
My only problem with this monster is it's a mouse vaccuum cleaner (I named him Hoover) and makes quite a mess. Spot cleaning isn't awful (but the spots are quite large), but his output is huge compared to my other animals, so full cage breakdown and cleaning is needed more frequently.
I think basically the majority of beginner snakes would be decent choices. Snakes make ideal classroom pets compared to lots of the other beasties.
Alot of people said corn. I'd have to agree. Never owned one, but I understand they are pretty active (like my mexican black), easily handlable, not too big, and pretty cheap. Plus the colors are fantastic. Heck, just a typical albino is pretty impressive.
If you want cheaper, try a brown african house snake. Although they tend to hide alot. On the plus side, they are remarkably tame (except when you try to hold them behind the head).
Just out of curiosity, where are you situated? As ChrisH said, a local snake might be a great learning experience.