The coach has poor social skills. In his defense though it is likely the kids have no other courts available to them. The coach is filling a void and is actually providing a public service by engaging these kids in some useful constructive activity. Better to be on the tennis courts than to hangout at a street corner skate boarding, smoking who knows what else. I would have confronted the coach on his manners and just replied to him something like "okay easy, you don't have to get nasty about it" As far as the poor tennis technique you noticed a lot of coaches don't know how to hit the ball correctly themselves and even if they do, it's really hard, try teaching a large group correct technique sometime.
Wilson PSC6.1, but lookin for a Tweener.