Legal Question in Criminal Law in Texas

It would be an assault charge. I assume that it caused pain (which equates to "bodily injury" under the penal laws.) So, it would be a class A misdemeanor which means up to 1 year in jail & up to a $4,000 fine if convicted. (Deferred and straight probation are also options.) And, he would not be able to teach again.

You are assuming that the police will take charges, though. My guess is that the teacher's story will be very different from your son's story. And, there may be other witnesses, too.