I know you already said you directly told her this but I think you should send her this message. “Hey, (whatever her name is). I’m really uncomfortable with the sexual messages you send to me. I know sometimes I play a long but it’s because I don’t know how to react. I just want to tell you that I’m not romantically interested in you, and neither am I sexually interested in you. Please don’t text me things like this on further notice. Let’s just keep this casual for school.”
It’s clear, it’s not extremely rude, and it isn’t giving her any signals at all.