Bit overwhelmed with all the links, but from an initial look my first reaction is that you need more light on the screen, and while you're at it, a backlight on your subject would help things along too.
From the fringing it looks like you need to turn up the gain of your key. If your key effect doesn't have a screen gain control, you can achieve the same ...
Large area soft lights. Bouncing lights off the ceiling is a good idea. If you don't have soft lights, you can soften them by placing a diffuser in front of them. Shower curtains work well for this. (Be aware of fire risks if you're using hot lights).
Good explanation of hard vs soft light here:
Shower curtain diffuser:
Did you try bouncing the light off of the ceiling? Make sure none of the light is visible in a direct line to the blackboard. You will need more light to get the same exposure, but, if you don't have more lights, more exposure might still work.