What was the Supreme Court Case Tinker v. Des Moines about?

Answer: The 1st Amendment

Mary Beth Tinker was a 13-year-old student who wore black armbands to school to protest America's involvement in the Vietnam War. The principal forbid the students to wear black armbands after a couple days of the "protest". The case established that freedom of speech must be protected in public schools if the show of opinion is not disruptive to learning.