I take it that your question is in response to #1. Many people feel it's a choice and some a mental illness. I personally feel in some cases it's a choice. I.E. When you have a gay man dating another gay man dressed as a woman. That would indicate that he actually has an attraction to women in which case it's a choice to be with a man. Same goes for lesbians. When one is a woman and the other dresses like a man it's a choice because obviously the woman is comfortable with men enough to date a woman that looks like one. I've known bisexuals and clearly they have a choice. Regardless it shouldn't be taught in the classroom unless it involves reproduction and sexually transmitted disease which falls under biology and health classes. It's not the schools place to say what is acceptable sexuality and what is not, that's up to family's and government. If the child disagree's with his family he can do as he likes when he turns 18. If he disagrees with the government he can work to change things at age 18. It's only the schools job to teach tolerance to all and prevent bullying.
I had a lot of gay friends in school and bullying was the biggest problem. Of course back in my day the school didn't do much about it and that was wrong. I knew one kid that had to quit school, ones that had to hide behind a "girlfriend" or "boyfriend" and another that was in denial. IMHO if the school had done more to teach tolerance than their school years would have been much better. The school only did something when it endangered them physically on school grounds. That left the job of protection up to other students to defend against bully's. It sucked.
Schools should not have to teach tolerance and respect for other. However when parents fail to do so, schools become the last and only resort.
Being homosexual or heterosexual is not a choice. Sexuality is how you feel and think about members of the same or opposite sex. It is who you are. You can avoid having sex but you can't avoid thoughts, feelings, desires, and unconscious gestures. These things are what determines your sexuality.