When you have students do that drill do you have them kicking with the points of their toes? I believe that that was the issue, not the drill itself.The front kick to a horizontal paddle is a very common drill. To me, it is a warm up for roundhouse kick, as opposed to being a front kick exercise. If it were me, I would have let it go at the time, because no one was in any immediate danger from doing that, and I would have discussed it with him afterwards.
Daniel