The lowly overhead projector and a decent LCD flat screen and a cheap computer can provide the low budget school with some very fancy teaching tools.
The overhead projector needs to be at least 2500 lumens, otherwise it will be too dark and you'll have to turn off all of the lights in the room. This is not a good idea at any age K-12.
The LCD projectors can be found on Ebay for $150~300. Try to find one that supports RCA VHS input and output as well as an active screen matrix 800X600.
You'll probably want a sturdy cart too with an under carriage to put your PC. The PC can be an older one as long as it has a NIC card for network access. Get long extension cords for power because plugs in school rooms can be very far away from the action.
Here's a graph of the setup