I don't remember if Momo-chan-sensei's age has been stated, but she seems rather young. Not too many years since she got out of the uni. She would absolutely be included in the dreams of many a high school boy. Biologically there's nothing strange about a 16 years old guy being infatuated with a 25 years old young woman. There's a different sort of charm between such a dream and someone of your own age. The social order in developed countries just doesn't view it in good light, for a good reason, though ironically that reason is a part of the attraction (taboo).
In my time, I had a couple of young teachers, albeit briefly (temp teachers). They were treated differently compared to the older ones. Just like in this chapter Momoko is called Momo-chan. No way they would call some 40 or 50 years old teacher that. Those cute teachers I witnessed were trying to be 100% professional, though, always. They were teased a little bit, however (not sexually). I guess it's inevitable.