Loli characters are a pretty disgusting trope and I completely agree. The presence of a bubbly, scantily clad 10 year old is usually enough for me to drop something immediately.
High School age characters are different though. I don't really see the problem with depicting them with fashionable, somewhat revealing outfits. That's completely realistic to what actual high schoolers wear. It's bad when it gets hyper sexualized, but that's true with adult characters too.
I also don't see the issue with them being in romances with other high school aged characters, as is usually the case. Like Rikku can flirt with Tidus a bit, and that's fine. They're in the same age group. Yuffie on the other hand, who is with all older characters, doesn't have any flirtatious scenes. Which is good, because a character in their 20s hitting on a 16 year old is a bit weird.
High School romance stories are very popular in Japan, but they're popular in the West too. Countless shows like Boy Meets World, or any number of ABC Family or Disney channel shows center around teenagers and romances, and they're all fine because they aren't tastelessly sexualized. You also have shows like Riverdale, where all the characters are very sexual and overly mature, which is also a common trope in Japanese games and anime. Where you have teenagers basically being adults and young adults being experienced older adults.
I think a lot of this is because it's aimed at teenagers who want to think of themselves as competent adults, and at adults who want to reminisce about being a teenager, but don't want to acknowledge that they were dumber back then.