I really don't think homosexuality should be discussed in any more depth than a brief reference when teach about contraception. I think making the student body of a school more aware of homosexuality and how there's no choice could improve difficult situations for some gay teens.
However when sexual health is taught, students are at a very confusing and hormone driven time in their life and making them aware and promoting homosexuality as 'normal' could make them believe they are gay.
This could lead to a sudden increase in 'out' gay teens whether they are genuinely gay or not. (Which sounds good to me!) but on the other hand this increase in gay and Bi teens would do pro-creation no favours, not to mention send the contraction of STI's soaring.
Maybe that's thinking into it way too much
All I'm saying is, these educational changes are very sensitive and need to be dealt with appropriately at an appropriate age. Or else they could have the opposite effect and make being gay the 'butt' of even more jokes!