Summary:
Green screen work is not trivial, the quality largely depends on technique and good lighting, and at least enough space for doing the work you suggested. It does require software although iMovie bundled in most MACs has a chroma key effect (the software that allows you to composite your subject with whatever background you want including maps, other videos, pictures etc).
Things you will need to purchase:
A pro screen.
Enough lights (with stands and diffusers) with the correct color temperature to fill the screen evenly as well as light the subject appropriately.
For your situation I think you might get there on $200, assuming you have iMovie on your MAC, spend under $100 on a screen, and another $100 on enough lights do do the job.
Details:
Here is a good tutorial on how to shoot with a green screen:
http://www.youtube.com/user/tubetape?v=q3PZO_lCBkw
Here are some tutorials showing you how to composite your green screen with iMovie:
Things to remember:
1) The green screen has to cover the entire area of the frame you are shooting. That is you can't have the green screen half way in the shot or showing any edges.
2) The green screen has to be flat, no wrinkles, no reflections, no hot spots, and no shadows.
3) The green screen has to be lit evenly and bright enough to register light in the frame.
4) The green screen should be at least 5 feet behind the subject and appear diffused not in focus. (The subject should be in tight focus, and the background green screen out of focus to optimize how it blends in the chroma keyer in your editing software).
5) Your subject can not wear anything green and should not have anything reflective on them, e.g. jewelery, watches, glasses.
As you can see from the tutorials a 6 x 9 room may be too small. Alternately if you purchase a portable screen, you can clear the classroom, move your lights in, set up the screen and you are good to go. Tearing down the set should take even less time. Better to have plenty of space to do this right then jam this into a space that doesn't allow for the minimum screen to subject distance.
Alternately, if you shoot outside with indirect sunlight or on a cloudy day, you won't have to buy lights.
Suggestion 1: Take it in two steps. Get a screen first, shoot outside when the light is flat and diffused to get the hang of it, then spend the money for lights and go for the studio thing.
Suggestion 2: Should you use the portable set it up as you go method, once you achieve optimum results document the studio set up, where the lights are placed and aimed, where the subject is, and so on so you can repeat this next time.
