A case of some teen just doesn’t want to be told what to do. If I remember correctly, not only girls, but boys also weren’t allowed to wear tank tops or expose their shoulders (in school) during my school days. The only exception was basketball jerseys during basketball practice or games (for both boys and girls).
There was no explicit dress code in my days, the wonderful 90s, but I think both boys and girls exercised caution with regards to school clothes. How you dress in your private time, and how you dress at school are not the same. I think asking either gender to cover up their shoulders or wear shorts, skirts, etc. that are at least mid thigh length is reasonable. If the school dress code is applied to both genders, how is it sexist? Authoritarian? Maybe. It’s a school, not some local teen hangout.
I hope the teachers and school board put their feet down firmly on this one. I personally would prefer uniforms if I were a student again. It’s definitely better for an environment of education. However, I also think students should be happy to have the freedom to dress as they please (within the school dress code).