The student should prevail, but only if she can show that school has already allowed other religious or political messages.
Otherwise, the school should prevail, as it has the right to set reasonable boundaries. Because if a Christian message is allowed, so would an Islamic message.
And I would not want children exposed to Muslim propaganda. Remember, we’re talking children here. Even in high school, they can be quite impressionable.
She doesn’t even have to show that it’s a political or religious message previously allowed. She just has to demonstrate that anyone can decorate their space. The school does not have a right to prohibit religious or political expression. That is pure First Amendment, and there is no exception for schools according to the courts.