Why Has the Supreme Court Allowed Restrictions on High School

Why has the Supreme Court allowed restrictions on high school students' freedom of speech?

A) As with the establishment and free exercise clauses, the First Amendment does not apply to citizens under 18.
B) Public and private schools typically require parents to authorize violations of students' civil rights.
C) Public schools are governed by local school boards, and the Supreme Court has not incorporated the Bill of Rights to apply to these local government organizations.
D) Schools have an educational mandate to teach socially acceptable behavior.