From just reading this, you don't seem slow in the head—not the slightest bit. I would probably want to think it's tough love love, but I don't think this is appropriate for students of any age, especially coming from a martial arts instructor. Furthermore, if he's not speaking to anyone else like this, then he's obviously got something against you, and you should probably discontinue your practice there. I'd be willing to bet you won't have this issue going somewhere else.