Indeed, something similar happened the day of the 2010 incident that led to the lawsuit. After the principal 2010 ordered the students to change their shirts (or to go home with an excused absence), the students got threats of violence:
“In the aftermath of the students’ departure from school, they received numerous threats from other students. D.G. was threatened by text message on May 6, and the same afternoon, received a threatening phone call from a caller saying he was outside of D.G.’s home. D.M. and M.D. were likewise threatened with violence, and a student at Live Oak overheard a group of classmates saying that some gang members would come down from San Jose to ‘take care of’ the students. Because of these threats, the students did not go to school on May 7.”
The court therefore concluded that, under Tinker, the principal’s restriction of the students’ speech was permissible:
“Here, both the specific events of May 5, 2010, and the pattern of which those events were a part made it reasonable for school officials to proceed as though the threat of a potentially violent disturbance was real. We hold that school officials, namely Rodriguez, did not act unconstitutionally, under either the First Amendment or Article I, § 2(a) of the California Constitution, in asking students to turn their shirts inside out, remove them, or leave school for the day with an excused absence in order to prevent substantial disruption or violence at school.”
This is a classic “heckler’s veto” — thugs threatening to attack the speaker, and government officials suppressing the speech to prevent such violence. “Heckler’s vetoes” are generally not allowed under First Amendment law; the government should generally protect the speaker and threaten to arrest the thugs, not suppress the speaker’s speech. But under Tinker’s “forecast substantial disruption” test, such a heckler’s veto is indeed allowed.