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Perhaps my favourite movie ever.
Probably the only good remake of a Japanese horror movie, too.

Gore Verbinski

I don't mean to start a huge argument, I'm really wondering. and I see the show isn't ecchi or whatever, so that's good. I just wonder where the line is crossed into technical "loli." Like sure, the show says they're 15, but they look tween-ish.
Well, it's always a grey area when you're talking about a show that features high school girls because it's animated not live action. You can analyze looks, but that varies with art style and, just like in a real life, people don't necessarily look their age. Then there's context, but, as you said, that can be tricky as well.
In this case, while they might appear younger than 16-17 years old, they're never portrayed as being anything other than normal high school girls. So...*shrugs*