The ban includes T-shirts and other clothing that have replicas of the flags. At least for now, it also includes Mexican or American flags flown from the antennas of cars parked on school property.
The ban takes it as a given that the Mexican and US flag are equal. They are not. The US flag is the flag of this Country. The Mexican flag has no more standing here than the Jolly Roger or someone waving a dish towel.
Those students and others who marched under the Mexican flag should march on down to where that flag means something and stay there.