The coach must've really grabbed him hard by the neck to justify an assault charge. I'm having a hard time picturing the scene, myself. It's hard to do any damage with one hand on the back of the neck, unless the person doing it is out of control. Was the 17yo(I refuse to call him a kid) hurt? Was he shoved to the ground?
As someone who coaches at younger ages myself I have had to educate myself on certain things like spotting concussions, and how not to do certain things that can be taken the wrong way such as having private meetings with a player, or driving them home after practice. Laying hands on them would be right up there as a no-no...hard to believe this guy was ignorant of that, especially seeing as he was a teacher, also.
I have some sympathy for the coach, but he handled this wrong. Neither side covered themselves in glory on this one, but again, without seeing it I hate to give to much of an opinion. I do know that arguing with umpires is unacceptable at our HS. It still happens(rarely), but the player is VERY conscious that he is on thin ice the longer he questions a call. Players can and do get benched for this.