we got it...the teacher is a Cold fish...
big whoop!
just put him among the most expressive people in the world and you will have a great story to tell...
and its obvious, some girl fell in love with him and was his student, got yanadere on his ass, he had to put her down.4
this other Male Sensei shown in this chapter, receiving those message from his student about going out with him is an obvious hint.