I'm not a huge anime fan, but I have seen some anime and western animation that takes place in highschool, or highschool age, so I guess I get to have a casual opinion.

And that opinion is that when I see animated characters in highschool I don't think of actual highschool kids, because highschool kids are gross children. Similar to how Hollywood will have a TV show about teens and cast a bunch of 28 year olds to play them, there's not really any connection or association in my mind between these characters and real youths I might see on my walk to work or on the bus.

The reason that highschool keeps coming up is because the setting has a lot of constraints and dynamism. True children have no agency in the world, and adults are set in their ways or part of the system, whereas teens are becoming people, and so there's a question about who they'll become. There are adults and structures that have some level of authority over them, which can make the stories have conflict, but those adults don't hold ultimate authority like they would with toddlers or something, so there's a chance of victory. This is why even shows that aren't about highschool, like Avatar the Last Airbender, still feature kids in an age where if there was highschool, they'd be in it. Or fantasy adventure + "highschool" like Owl House. They're stories of self-discovery that most adults don't experience because they already self-discovered and they're through that now.

So my answer is that I don't want to convince you of anything. If, when you see a character that's meant to be in highschool, you feel like that is a highschool person in real life, that's fine. And I could see that being creepy. But when I experience an animated character they don't feel like a real person to me, they're an abstract sketch of a person, lacking in detail, and one of the details they lack is "actual organic age and the implications of the body characteristics that come with it". Instead they're just "confident" or "awkward" or "shy" or "bold, but reckless" or whatever.

There's nothing about them that feels anything like any real person I've met or seen.