Pledge Refuseniks Suspended
The ACLU says that's not constitutional. The ACLU is correct.
The school does not compel the saying of the pledge but it does compel standing for it. But any forced demonstration of respect for America by a government school is unconstitutional.
However, private schools may do so. As well as private businesses and organizations as a condition of belonging to them.
Recall several years ago, former NBA player Chris Jackson (aka Mahmoud Abdul-Rauf), a Muslim, refused to stand during the national athem during games and he was suspended. He claimed the American flag was a symbol of oppression. Blah, blah, blah.
After a one-game suspension he agreed to stand. Apparently his paycheck was more important to him than demonstrating his hatred of his country. (Today he runs a mosque in Louisiana.)
If these kids had brought an American flag to school and stomped all over it, they could have been legally suspended, not for being unpatriotic but for being disruptive of school order. The same as if they, say, started chanting anti-gay slogans. Such speech would be protected on the street but not in public schools.
Still, silent inaction during the pledge of allegience is not actionable by government authorities and that includes public school bureaucrats.