This film tries to be sophisticated but it doesn't make it. As a secondary teacher, I can say that both teachers don't maintain a strong enough boundary with the two female students, probably all their students. It is a far too casual - I wondered for a while if it was a community college! You can have a bit of repartee with students, you can have some connections. But you've got to make clear your interactions with colleagues are different from your teacher-student interactions.
That said, apart from the kiss this teacher did nothing wrong - just ill-advised. Whatever feelings a teacher might have, they aren't wrong. What's wrong is acting on them. The colleague asking "Are you in love with her?" was not what the situation required.