Part of being a teen is rebellion, I don’t know a single person who hasn’t rebelled (might not be as drastic as an outwardly flaunting). Girls in particular who rebel are looking out to see who’s watching and how they’ll react.
They need unconditional love, I can’t imagine that wearing pants will make you (a teacher) cringe yes. but try to show respect and not be shocked. WE sometimes like the shock value of look I can cross lines, and yes! I saw my teacher and looked like _____. I would only say that a good teacher (someone I had, would wish on e/o else would say omg, how are and ignore the dress. The comments aren’t necessary since she prob. knows you didn’t approve.
When you’re talking about an adult it’s different, but come on teens will be teens!