Personally I'd suggest a cornsnake or milksnake. Ball pythons being so much shyer and far more nocturnal and with thier tendency to be in their hides a lot, well I don't see them as being a great classroom pet. Granted they are nice and beefy so easier to hold on to than a fast moving colubrid and tend not to be a bite first, ask questions later type of snake but still, not my first choice in a classroom.