“Why are they teaching that in school?”
Because public schools are in the business of destroying the value system that you gave your kids. The school system wants them at an earlier age, so the conversion can last for a longer time and turn them into despicable intolerant liberal morons.
Schools have control of your kids’ waking hours than you do.
The Marxists know that if you sexualize children at a young age you can destroy their faith in God. To be replaced by faith in a Marxist government instead.