This case is not about a political debate over the criminalization of drug use or possession. This is about whether a principal may, consistent with the First Amendment, restrict student speech at a school event, when that speech is reasonably viewed as promoting illegal drug use.
Principals have a tough job. When Morse saw the banner, she had a split second to decide what she was going to do about it. It was reasonable for her to conclude that the banner promoted illegal drug use- in violation of the school policy.
Tinker v. Des Moines Independent Community School District held that student expression may not be suppressed unless school officials reasonably conclude that it will "materially and substantially disrupt the work and discipline of the school."
Tinker v. Des Moines Independent Community School District
School boards know that peer pressure is "the single most important factor leading schoolchildren to take drugs" and that students are more likely to use drugs when the norms in school appear to tolerate such behavior.