Unfortunately screen-top tank + heat lamp = low humidity. You can fix and fix all you want but you're fighting the laws of physics.
The other posters have given you some good options. I would add:
- Get a plexiglass top to put over the screen, with a hole cut in it where the heat lamp would go. It's not expensive and there's less of a fire risk than putting cardboard or fabric close to the heat lamp, especially if your students will be handling the snake and its setup.
- Make a humid hide. An inexpensive option is to get a plastic food-storage container large enough for the snake, cut a corner off of the lid for an access hole, and fill the container with damp spaghnum moss or put in a layer of damp cocoa-husk. The downside to this is that every snake I've provided with a humid hide also uses it as a potty, so it needs cleaning often and the snake can end up messy as well. The upside is the rest of the enclosure stays clean.