I personally cringe at how both girls ignore the simple fact that student-teacher relationships "in that way" might spell trouble for the teacher, if word gets out. And one takes it further by trying to entrap him.
In The End, it could be said that Sensei was tempted past his limit by young teen "T & A", so that sexy time was inevitable. And after that, the girl who was originally engaging Entrapment jumps at the hot, young assistant teacher who shows up at the end.
In the end, I guess it's pleasant enough if a fap-n-go threesome is your thing.