the teacher already lost the fight by having the kid in his face, squared up.
you NEVER let it get anywhere near that point. You have that kid in the office with a referral when the dirty looks/arguments start. And you always have a heavy enough object nearby for a KO, should something crazy happen and a kid blindside ya.
Throwing haymakers after squaring up, that dude needs to quietly leave teaching, that's just embarassing in my opinion.