Read this article from CBC.ca first.
Some teen 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, 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 longer shorts or skirts is reasonable. If the school dress code is applied to both genders, how is it sexist? Authoritarian? Maybe. It’s a school.
I hope the teachers and school board put their feet down firmly on this one. I would prefer uniforms if I were a student again. It democratizes the school environment. There is less focus on fashion or display of wealth, and more focus on education.