Can a public high school ban a certain brand of clothing because they think it is gang related

Answer

They are doing it all the time. Los Angeles had Gang Injunctions, prohibiting Gang members just from hanging out with each other.

See Calif. Street Terriorism Act - CA Penal Code 186.21

http://www.leginfo.ca.gov/cgi-bin/displaycode?section=pen&group=00001-01000&file=186.20-186.33

For other information see www.SteveShorr.com/Criminal.law.htm

Answer

Yes, as long as it is rationally related to a legitimate governmental purpose.

If the government's purpose is to curtail gang-related action, and banning certain clothing is a way to do so, then there is a rational relation that no court will overturn.

Answer

Public schools can implement dress codes and/or ban certain apparel, it is not considered a civil rights violation. However, the parents or guardian of those students who object to the rule may file an appeal on the minors behalf in the circuit court of jurisdiction opposing the regulation(s).