I'm going to say a word in defense of the school's negative reaction to the t-shirt. As someone who taught for years in both public and private schools, I have no difficulty seeing why the t-shirt is a problem. I also can say that few schools would not react. The question as to *how* they should react is valid, though.
The problem with the shirt is the sexually suggestive wording in "yummy boys". Imagine if the shirt said, "I dig yummy girls" instead. Would we then see why girls who saw the shirt might feel objectified, and therefore offended?
Consider a more egregious and real life case, in which a girl wore a t-shirt that mimicked the Coca-Cola logo with the message "I like vagina". The problem wasn't that she wore a pro-lesbian t-shirt, but that it sexualized her identification as a lesbian. One message that GLBTQ advocacy groups emphasize is that one's sexual orientation is about far more than sex. These t-shirts only perpetuate the myth that being gay or lesbian IS all about sex. I doubt we would allow a boy to wear that message, so we don't give a pass to a girl who does.
If Luke were wearing a shirt that affirmed gay rights, he would have a case that the message is protected (political) free speech. If the school objected to a shirt with a rainbow flag and a message of support, then I'd call the school out of bounds in a heartbeat. Having said that, I do have to remind everyone that conservative courts have given schools a great deal of leeway with dress codes. If they can justify a decision on the basis of a clothing article being disruptive or provocative, the courts will probably back them up. Before challenging a dress code decision, I'd want to see clear discrimination or stifling of free speech without any basis.
That was a long word.