i dont really get the mindset in these shoujo mangas that theyre too young for sex in HS w...

Because adults, especially parents doesn't want to encourage teenage sex.

And there's definitely a regulation for publishing company about censorship and rating system. Shoujo manga is usually for girl from elementary to high school. I don't think it's right to show adolescence girl that's in HS sex is a common thing. Especially when they didn't even get sex education to have safe sex.