Although I'm a male reader who knows absolutely nothing about girls, I don't think groping each other's boobs is a common thing to do between female friends
Excuse me, not sure if is to keep it somewhat pure despite the actual plot. But normally the breasts are not as high as the collarbone.
Tho sensei has some serious acumulated sexual frustration to react like that for being barely touched with a flat palm of all things lol