[radioman]

What amendment is that?

Amendment 1 - Freedom of Religion, Press, Expression. Ratified 12/15/1791.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.

Amendment 10 - Powers of the States and People. Ratified 12/15/1791.
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.

Where does the Constitution delegate the power to the government to restrict free speach on a public road?
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[theDentist]

Where does the Constitution delegate the power to the government to restrict free speach on a public road?

Accdg to the SCOTUS ruling: 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. In Tinker v. Des Moines, the Court declared, in holding that a policy prohibiting high school students from wearing antiwar armbands violated the First Amendment, 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,”.

The banner was unfurled directly across the street from the school, and pointed in the direction of the school and students, therefore it was in the Court's opinion, a disruption of the school and it's discipline.