*Ahem* https://en.wikipedia.org/wiki/Tinker_v. ... l_District
"The court's 7–2 decision held that the First Amendment applied to public schools, and that administrators would have to demonstrate constitutionally valid reasons for any specific regulation of speech in the classroom. The court observed, "It can hardly be argued that either students or teachers shed their constitutional rights to freedom of speech or expression at the schoolhouse gate."
A girl showing her shoulders is not a constitutionally valid reason to have her freedom of expression infringed upon.