In which case did the Supreme Court rule that student armbands represented pure speech?

The Supreme Court ruled in the Tinker v. Des Moines Community School District case that student armbands worn in protest of the Vietnam War constituted pure speech. In this landmark decision, the Court upheld the students' right to express their opinions through symbolic speech as long as it did not substantially disrupt the educational process. The ruling emphasized that students do not "shed their constitutional rights to freedom of speech or expression at the schoolhouse gate." This case set a precedent for how student expression is treated under the First Amendment in the school context, highlighting the importance of protecting free speech rights, even when expressed in non-verbal forms.

In contrast, the other cases listed focus on different aspects of student expression and the role of school administrators in regulating speech or activities, not specifically defining student armbands as pure speech.