I think it's partly because grades and school simply don't matter prior to high school (in the US, at least). Theoretically, as long as you don't fail, you could barely scrape by in elementary and middle school and it would never make a difference in your life. School then is something you put up with, you "put your hours in" every day, but your performance doesn't really mean anything to anyone, so kids have all of this pent-up energy during school hours and nothing worthwhile to put it into. Into social hierarchy bullshit and bullying it goes. There were definitely instances of bullying/power hierarchies in my middle school, but for the most part kids were just indiscriminately dickheads to each other because we had nothing better to do in our boring little world.
Once you get to high school, your grades actually matter for deciding the course of your life, so all of a sudden you have to try in school if you want to get into a good college. Three consequences: kids have less energy to waste during school hours, they actually have a goal (however meaningless) to work towards, and being a good student becomes cool (as long as you meet some baseline level of attractiveness and social skills) because (in an ideal world) it means you have a brighter future ahead of you than flipping burgers at McDonald's. At this point, most kids just want to focus on their "work" and enjoy their remaining time with friends, not trying to inch their way up a pecking order. The exceptions were the kids that knew they had special privileges (football players and cheerleaders with rich daddies), and the poor kids with the worst grades that knew their futures were bleak.
It pains me to say it, as a slacker that hated the "school" part of school much more than the social interaction, but maybe making grades "matter" earlier on would improve the quality of social interaction in middle school. I don't think that's worth wasting more years of the lives of children, though.
Two other non-age related factors: American middle schools encourage microcosms to form by generally being smaller than high schools, and by having tighter-knit class units (at least in my school, you stuck with the same kids for everything except electives). Among the first things I thought when I started high school was "there is no way I could ever meet all of these people," later followed by the realization that no one else gave a shit about strangers anymore either.
Once you get to high school, your grades actually matter for deciding the course of your life, so all of a sudden you have to try in school if you want to get into a good college. Three consequences: kids have less energy to waste during school hours, they actually have a goal (however meaningless) to work towards, and being a good student becomes cool (as long as you meet some baseline level of attractiveness and social skills) because (in an ideal world) it means you have a brighter future ahead of you than flipping burgers at McDonald's. At this point, most kids just want to focus on their "work" and enjoy their remaining time with friends, not trying to inch their way up a pecking order. The exceptions were the kids that knew they had special privileges (football players and cheerleaders with rich daddies), and the poor kids with the worst grades that knew their futures were bleak.
It pains me to say it, as a slacker that hated the "school" part of school much more than the social interaction, but maybe making grades "matter" earlier on would improve the quality of social interaction in middle school. I don't think that's worth wasting more years of the lives of children, though.
Two other non-age related factors: American middle schools encourage microcosms to form by generally being smaller than high schools, and by having tighter-knit class units (at least in my school, you stuck with the same kids for everything except electives). Among the first things I thought when I started high school was "there is no way I could ever meet all of these people," later followed by the realization that no one else gave a shit about strangers anymore either.