Definitely chat to the principal if you feel the teacher hasn't addressed your concerns or responded to the incident appropriately.

I will admit I feel less outraged now I know it was her outer pants. I interpreted pants to mean knickers, which IMO would be more serious & warrant immediate contact. However it's still not ok to have heard about via another parent first, and there needs to be more communication around what response the school/teacher has taken in relation to the disrespect shown to your DD's personal space.