Agreed, but I think Kuon realized both that his words would hurt sensei more than a punch and that the consequences of punching him would only make Nanao feel worse. At the very least, punching a sensei on campus in broad daylight would give sensei leverage over Kuon. If it was brought up to the campus disciplinary committee or the police, what happened to Nanao would be dragged into it.
Ah, well, in terms of common sense, I meant more for my own satisfaction as the reader than feasibility. I get frustrated as a powerless onlooker sometimes, although I'm sure you can relate. :'D
Honestly, as much as I like sensei's character... I really wanted Kuon to punch sensei at least once, just for Nanao. That was a really cruel thing to do to someone whose self-esteem has always been so low.