It's about koharu being a woman who dragged her past relation with man to meddle with other men ignoring the feeling of the those who infatuated with her. Basically principal thinks she's inconsiderate woman.
like the other commenter said, thank you for spelling it out for everyone.
Koharu is oblivious to men who are into her. This includes Tetsuo, Arisu, and Miyao (the friend that drinks with her and Doi sensei). Because of this she unknowingly causes pain to those around her, ironically what she was also a victim of.
So the principal doesn’t like her. When Tetsuo mentions that Koharu said her past love was an example to follow that pretty much validated the principal’s feelings and she even says “she’s exactly the kind of woman I thought she was”.
So the principal’s feelings are complicated. She respects Koharu as a teacher for getting Tetsuo to turn his life around. She hates Koharu as a woman for her being inconsiderate of the feelings of men around her. She hates Koharu as a mother for meddling with her son as well as toying with her son’s heart.
If you were a parent and your child’s heart was being toyed with (intentionally or not) and being led around, you’d probably have negative feelings about the situation.