so far the standard of whether something has been allowed or not is if the school can show reasonable harm or that it will have reasonable harm.
a student standing in class and reading bible verses would be a disturbance to the class and others. the school does have the right to maintain control.
the school in this case allows other groups to use poster boards and other items when they want.
this is why schools lose so many free speech cases because they cannot overcome that burden of proof.