So, what is the answer?
The school exists to teach children both the basics of education and to reinforce a specific set of values, that you clearly don't agree with.
That includes teaching that homosexuality and transsexuality are sins. I am overweight, and gluttony is a sin. I don't expect the church to start preaching that gluttony is not a sin, so it won't hurt my feelings. Even though science now says the propensity to be overweight is genetic.
It is true we are all sinners and as such we need a Savior. The Bible also says we inherit sin from the moment we are born and that the sin of the parents passes to their children...
For Christians, the Bible is their primary reference book regarding what is sin and what is not. It should be noted, Jesus came to the world to "save" sinners, so God does love sinners, but Jesus also never said sin was good or okay... So, the Christian is not asked to pass judgement on sinners, but neither is the Christian able to redefine "sin" to meet modern standards of right and wrong.
Look at your behavior, you call me homophobic, but if I am homophobic why would I have promoted numerous gay people to senior positions in the companies, I ran. Why would have knowingly hired gay people considering their qualifications as the criteria for hiring them, not their sexual preferences. and firing people who would not hire qualified gay applicants? In the 80s, when little was known about HIV and several employees asked me to fire an infected gay man, I suggested they could leave the company if they could not handle working alongside a gay man with HIV. In the early 90s, when I lived in Seattle, I intentionally lived in the gay community (lots of great cafes and art stores). You don't have to be homophobic (someone who dislikes gay people) to simply recognize the Bible is pretty clear homosexuality is a sin. I think adultery is a sin, but I am not "adultery phobic".
I accept people for who they are... sins and all... just as I hope they accept me. The story of Jesus and the woman caught in adultery is a great illustration of the point. Jesus did not say adultery was "okay". Instead, He said let he, who is without sin, throw the first stone.
The problem these days is that whether you are Christian or not, people feel empowered to judge others rather than simply identify something as a "sin or not a sin" according to their beliefs. The real question is why you feel empowered to judge Christians instead of simply disagreeing with them.