Teaching tolerance violates civil rights
I'm not sure what ground this will end up being tossed, but I cannot see how these parents can win a case that claims that anti-harassment training violates their offspring's rights to verbally berate homosexuals while in school. If they do win, it will only be a matter or time before, for example, the KKK could claim their sons and daughters should not have to tolerate the black students, or neo-Nazis should not have to tolerate the Jewish kids. Black gay Jews would be really out of luck.
Personally, when I went to school, I went to learn. Sometimes that involved subjects I did not agree on, but that was okay with me. I agreed with some lessons and disagreed with others. That made me a better person and simply strengthened my beliefs.
No one is telling the kids that homosexuality is right or wrong, they are merely being told you cannot discriminate against anyone or assault them because of race, creed, or sexual orientation. Seems like a worthwhile lesson to me.
These students (or at least their parents) seem so afraid of learning anything that may contradict their safe and happy world they may as well not be going to school to begin with. Sit and home and have mom or dad feed you everything they want to tell you. Develop a job where you work out of the home and never have to face the real world.