As undeveloped mentally as highschoolers are, they have years more development more than middleschoolers

Middleschoolers are suddenly blasted with hormones, and are basically accelerating at breakneck speed. Everyone's body is changing wildly, you're thinking and feeling new things every day, and it's all a total mindfuck.

By highschool, the hypernovelty has worn off. Yes, they're still developing and figuring things out, but it's not quite as much of the surreal existential body-horror that middleschool is.

There's a slump in classroom performance and psychological well-being around 6th-8th consistently-observed enough that it's got one or more names in the fields of education and psychology ("middle school slump", "middle school malaise", "middle school plunge", et c.) Mention it to a teacher and they'll likely know what you're talking about.