what he did is NOT discipline. If the child is struggling in a area you help them and give them extra work to understand it. you don't call them names and hope that will get them to try harder. it will just have the opposite effect.
Isn't that what some of the asians do? "You are too dumb to go to high school". A lot of suicides happen around that point in peoples lives, I hear over there the rate for people put in that situation is quite a bit above the baseline.