How does fortas defend the majority opinion that free speech in school is constitutionally protected?

a. by giving examples of instances when the freedom of expression of students was left to the discretion of the principal

b. by arguing that each institution, including a school, has a right to decide whether or not to grant students constitutional rights

c. by explaining that neither students nor teachers have a right to freedom of expression while on a school's property

d. by citing a similar case, west virginia v. barnette, in which the court decided that students are not required to salute the flag