[T]he refusal of students to participate in the Pledge -- unless their parents consent -- hinders their parents' fundamental right to control their children's upbringing. The rights of students and the rights of parents -- two different sets of persons whose opinions can often clash -- are the subject of a legislative balance in the statute before us. The State, in restricting the student's freedom of speech, advances the protection of the constitutional rights of parents: an interest which the State may lawfully protect.