Final answer:
Justice Stevens argued that student speech should be protected by the First Amendment unless it breaks specific rules or advocates for illegal, harmful actions. The banner in question did neither, implying that the school should not have punished the student.
Step-by-step explanation:
Justice Stevens, in his dissent regarding the court's decision supporting Principal Morse, asserted that the First Amendment should protect student speech unless it explicitly violates a rule or promotes illegal and harmful conduct. According to Stevens, the student's speech in question did not meet either of these criteria, thus it should not have been restricted by the school.
In his opinion, the school's punishment of Frederick for expressing a different viewpoint was a violation of his First Amendment rights.