Two things:
1. Am I crazy or does she seem like she has a bit of a crush on the teacher? (drawing a teacher in a flattering way is not quite common) That would eliminate her as a gf.
2. I love some good translator notes and I appreciate not translating some things that make more sense untranslated and then explaining it later. Sometimes it's really just better to teach a word (I imagine a lot of us already know about the many variations of yabai, but some seem to think it's just used as a negative thing, so the explanation is warranted).