If your subject is to the side of the frame and is a close up, try a junk matte, which is a generic mask around your subject to minimize the BG (background) colors. If you can minimize it to one color (chalkboard), you’re good to go. Also, one thing that might help pulling the key is to bump up the “levels” on your footage, pre-comp the footage with your new levels, then keying out the green. Chalkboard green is really dull, hence suggesting bumping up levels. Once the key has been set, you can make a matte out of it by having the normal footage (the unmodified levels footage) replace it. All in all, it would be nice to take a frame and post it for more specific help.