It kinda sounded like the school had planned a Mexican-American appreciation sort of thing and I'm kind of getting the feeling that these kids were troublemakers and intended to cause something by wearing the shirts. Sure that doesn't exactly permit the school to tell them to flip their shirts or send them home, there's always going to be kids like that.
However, if they wore their shirts just because they want to, not intending to start a problem, then more power to them.