At the start of the year my daughter's teacher brought in a betta -- in a goldfish bowl -- for the class to look after.
After speaking with her about not keeping fish in bowls, I donated an Explorer 2.5g with filter, heater, syphon, etc., for them to keep Mac in. Mac loves his new home and is a very lively class fish. The class loves him as well
My husband promised the teacher that WE would look after Mac, so either he or I have been going to the school once a week to clean and inspect him. It's a bit of a pain, but I wouldn't not do it
I would suggest a betta, because they have a great personality. Another possibility is an Endler's Livebearer. Even if you go with a coolwater fish, you should still use the heater to keep the temperature stable. It can get pretty cold at night, over the weekends, and holidays. I don't know of any schools that don't turn off the heat when no one is there.
Great that you want to introduce *proper* fishkeeping to the class. Kudos!
Roan