Differs from place to place, I can not tell you the exact year. It probably started in 2015 or even earlier and has been gaining major traction in many American and European countries since 2019.
Discussion
Which schools? Because states have different curriculums.
Do you want me to name exact schools? I am afraid that is too much to ask. If you genuinely are interested in this subject I suggest you do your own research. You can also check out this article comparing Sex Ed curriculums in Canadian provinces. It is from 2015 and they already had gender identity included at that time. https://globalnews.ca/news/1847912/sexual-education-compared-across-canada/
Here are a couple examples of Sex Ed from UK and Switzerland.
