I don't like the idea of such a skill. I'd rather go along in a direction Stefan mentioned for the usage of skills sometimes, namely that you can get better by practice, and I believe this could be added by means of some simple modifiers, but what do I know
. In effect, it would add a certain kind of "distractibility" to a char, and the better this character's magic training is, the lower this distractibility would be. So a heavy duty spellcaster would be really hard to distract even if he was hit. Maybe a successful hit could increase the possibility of a distraction a bit.