My theory: In elementary school, the teachers and the administration don't make assumptions. Thy give the kids rules they understand. The kids have jobs. They know where they fit and what today will bring. They know the schedule so well that they can tell a substitute teacher what to expect.
They have one teacher, one classroom, one desk (home base), and one set of classmates. They are a _team_.
Then they hit 6th grade and start to move from room to room and teacher to teacher. They get a locker, but can't leave anything in it because they need to carry their backpacks from room to room.
And then they hit 7th grade. 6 periods (or 7). 6 or 7 different teachers, different classmates, different rooms, different _buildings_. Different rules. 4 minutes to run from class to class and Maybe have a chance to get to the restroom or your locker. No restroom allowed at lunchtime (seriously?).
Plus hormones beginning to rage. Bodies changing. Brains in flux. And the school expects much more from these kids. After all, they're not in elementary school anymore.
Is it any wonder that they melt down?