This prohibition was probably an overreaction on the part of ignorant school officials. I shudder to think how many of those I had to deal with when I was a public high school teacher.
It wouldn't surprise me but that this was a pretty beaded necklace with a cross suspended from it which some doofus took to be a rosary.
But then, I knew of schools that prohibited students' wearing professional sports team logo clothing at one time because some has been used as gang uniforms. This is probably similar.
If the girl actually was wearing a rosary as a necklace, then I agree with Costrel and MC that she hadn't a very thorough religious education. A rosary is not to be used as jewelry. Someone should advise this girl that wearing a rosary as a necklace is sooooo not done. Problem solved quite simply.
when my kids where in HS, Notre Dame gear was gang wear