Oh, a discussion about high school yuri without me? This must be fixed.
Gonna agree with most of Anon's points, if not all. Just add the factor of the lowest common denominator (not saying it's always the case, but there's too much of that) and the fact that it's possible that most females had their first same gender relationships (or things resembing them) during the highschool years.
Oh, that and the common trope of romantic friendships between two girls. Wouldn't say it's only an eastern thing.
** - cannot be 100% sure about that, feel free to lynch me if you disagree.
With that said, while I usually tend to choose tagged as adult life
, there's a lot of good yuri about highschool. It's just that for some (including me) it's not really relatable.