Perfect for differential equations, group vocabulary exercises, and stopping a 9mm round at close range.
It's sad that people even have to think of developing a product like this. I question it's usefulness though. Say a gunman does come in and you have one of these in the classroom. It would be very opportunistic to be near the whiteboard at the time, and have the reflexes to grab it and hold it in front of you prior to anything happening.
The shooter could easily just point and shoot somewhere else as opposed to the teacher. The whiteboard pictured only covers oh so much, and if the shooter had any intelligence would just shoot below it not to kill the teacher, but to take the teacher out of the situation. Which would delay the attacker, but I don't think the benefits are that strong.