Just say that homosexuality/bisexuality/etc also exists during the regular sex ed stuff. The "T", is a completely separate issue, I don't know where that would fit in. The question of why it all happens is far from settled - just mention that not everyone is straight and that's okay. 🤷♂️
My only other thoughts was that I'm skeptical that that site is a credible source of news, but after looking it up it does seem that that school in particular is having problems with pride events and angry parents in other sources of news, so it seems plausible.