Zero chance of this succeeding. SCOTUS ruled in the “Bong Hits 4 Jesus” case that students have no First Amendment rights.
Morse v. Frederick, 551 U.S. 393 (2007) ruled that schools have a compelling interest when the message is promoting illegal drug use.
Students are entitled to equal treatment under the law, and schools do not get to pick and choose what speech they prefer.
Schools cannot ban this student’s flag while allowing others to be displayed. They not only banned the student’s flag but also said they based such prohibitions on whether someone complained. That’s essentially an admission of guilt. Looks like a slam dunk case to me.
Schools can enact policies that keep order such as uniforms and restrictions on messages on clothes. But these policies have to be applied equally and not be used to target speech they don’t like.