I suppose a white board is better than nothing. I would prefer to have someone with a weapon to stop the criminals before they can advance toward the white board and shoot behind it. This method is still a defensive position and the aggressor, with a weapon, is still going to have the upper hand. It's not much different than hiding behind an object in the building. It may offer some protection for a short time. It certainly is not the best answer, in my opinion, but in some cases, it may be the best we are going to get.