Talk to some teachers who've been around for a while, and they'll give you an ear-full about how much middle school sucks. Something happens to kids around 6th grade, probably to do with starting to value peer-approval over authority-approval, and they start getting wilder. The work gets harder. Class-switching may start that early, depending on the school. Puberty. Kids from several elementary schools are thrown in with one another, in an abruptly-much-larger class with tons of strangers. A bunch of the kids develop mental health issues. A bunch start hurting themselves or get suicidal, and attempts may be made, some with, uh, "success". Drug use begins to enter the picture. All forms of bullying, including the "mean girl" type, get way worse in a hurry. It is obviously hell for the kids.

There's a consistent and widely-observed school performance dip in about 6-8th grade. I have a feeling our usual structure for schools for those age ranges, in the US at least, is in some way fundamentally and seriously wrong.