The trouble with saying "its a school, not a public forum" is that a school is, in fact, a public forum -- all educational venues are public fora.
Sorry, but no, they are not. I can't simply walk on to a K-12 public school campus and hang out there. I can't go into a classroom and demand that I be heard. I don't have the right to dictate what gets said in a classroom, or to even be there.
Viewpoint discrimination is a genuine problem, because even if the social value you want taught to your children is the one that says "gay is OK," that may not be the value desired by another parent.
But so what? The problem is that no matter what is taught, some parent is always going to object. I honestly believe that the public schools go to great lengths to avoid "teaching" anything that rational people are likely to find objectionable, or for which there is honest disagreement. Not everyone, though, is reasonable. The gay issue is a good example. You would think that no one would object to a school teaching tolerance for gay people. Given that gay students are routinely taunted, humiliated, bullied, terrorized, and even assaulted by other students, it seems like a mighty good idea for a school to try to instill some basic respect for the worth of all people, gay or straight. And yet, many people still object, claiming that to teach that it's not okay to denigrate gay people is to somehow promote the "gay agenda".
I could go on. Some people object to the teaching of evolution because they believe that to do so denigrates religion. Some people object to a literature class assigning kids to read The Adventures of Huckleberry Finn, claiming that the book perpetuates racist sterotypes. And of course some people object to teaching basic facts about human sexuality, claiming that if you merely point out to kids that condoms, when used properly, go a very long way towards preventing pregnancy, this will someone encourage kids to have sex. Just as a "heckler's veto" cannot be permitted, a few irrational parents cannot be allowed to "veto" a community's educational needs for its children. If the school district's cirriculum does not reflect the values or educational desires of community, then the community can elect a new school board. But you don't get to undermine the educational process on a school campus. If the school promotes a message of tolerance and respect for all people, you don't get to go on campus and promote a contrary mesage of intolerance and hatred.
Although I'm not among them, the fundamentalist Christians are right when they say our schools teach tolerance for anything but Christianity.
I'm surprised you've bought into this red herring argument. If virtually all of the students, teachers, and administrators of a school are Christians, do you really think there will be will be a lot of intolerance of Christians on campus? Maybe it's different where you live, but my observation is that it has historically been the small minorities in a community, not the large majorities, that suffer the "intolerance" of the community, since people generally don't discriminate against themselves. In an overwhelmingly majority Christian community, teaching "tolerance" for Christianity seems, to put it mildly, quite unnecessary. It would be kind of like if they decided that the overweight, handicapped, socially ackward, and and foreign kids were told in school, "Now, children, it is important that you show proper respect for the pretty and popular kids, because they're people too. We shouldn't tease them for being so perfect" If I were one of those kids, I would think to myself, "Wait a minute. I think they've got it backwards. We're the ones who get picked on. Why aren't they teaching respect for us?"
I sympathize more with the students than with any school administrator. When I was in high school I regularly war a black "biker" t-shirt on which the Jack Daniels label was emblazoned. Today, this would be considered disruptive and barred from the school -- did I or my contemporaries fail to become educated? I think not.
Well, I wouldn't have much credibility if I were to suggest that your Jack Daniels T-shirt, in and of itself, caused you and your contemporaries to "fail to become educated". However, I am sure you are familiar with the phrase, "The Death by 1000 Cuts". When institutions fail, it isn't usually due to one singular event. An institution fails because it slowly bleeds to death.
Our schools aren't doing very well, not as well as they should anyway. I believe that a large reason is a lack of respect by students (for which I blame the parents). A teacher, even an excellent one, cannot teach a classroom full of students who have no respect for the school, the classroom, or the teacher. So I think it's important for a school to demand some degree of respect from its students, since it's futile to try and teach students if you don't have that respect. And it doesn't seem unreasonable to me for a school to demand, at a minimum, that students not wear clothes that advocate or promote values contrary to the mission of the school. Since schools activel strive to discourage kids from drinking, it would seem to me that prohibiting the wearing of T-Shirts that contain the product logs of alcoholic beverages is reasonable. Not because kids wearing Jack Daniels shirts will cause the system to collapse, but because it's disrespectful to the institution. As a lawyer, I'm expected to wear a suit when I go to court. Not because the judge enjoys the way I look in a suit. It's because the judge is demanding that I show respect to the court, and the way you show respect to a court is by wearing a suit.