As the father of 6 years old twins (one boy, one girl) I agree with this 100%. My son needs more discipline than my daughter, he's more likely to get into trouble at school, he likes physical activities, etc. almost the opposite of his sister.
This does not seem like the right conclusion. More like "needs running around time and free play".
Cf. e.g. https://www.alfiekohn.org/article/discipline-problem-solutio... https://www.alfiekohn.org/article/beyond-discipline-article/
My parents did it. Sent to us to Catholic schools instead for many years. And we grew up on welfare.
Unfortunately this is effectively impossible to achieve for 2-3 months every year, with school in session and short Winter days. They're in lower elementary school but I think their total recess time per day is only about 30-40 minutes, and much of the time they don't even get that because it's raining or too cold (pft, whatever, wear coats) or something else, and they have them watch a movie more often than not in those cases (nb. this is a "good" school district)
We have one other kid—a girl—who doesn't seem to need this to not "act wild".
The other trigger for "crazy" behavior seems to be when they're presented with unpleasant social situations and don't know how to deal with them. Like, say, being forced to be around kids who are mean to them. Gee, I wonder where that happens a whole bunch.
On the whole I've been fortunate in life, but it's not an exaggeration to say my life would be dramatically different if I'd learned what was going on as a kid. Instead it was interpreted as a character flaw: that I just wasn't applying myself. Even though I know that's wrong now intellectually, emotionally I still had that message beat into me for years as a kid, and that doesn't just go away sadly. I'd be particularly vigilant about teachers creating that dynamic, as sometimes they do it with good intentions but fail to understand doubling down on a "tough love" approach will utterly backfire here.
The same amount of PE, along with became optional through junior high school and high school.
In elementary school, recess/lunch were required to be spent outside (unless it's -30C or so)
You could spend up to 15min at a time inside to warm up, dry off your mittens, and maybe grab a hot chocolate but after that you were kicked back outside
They crave (and physically need) hours every day spent running and climbing and swinging and rolling around, chasing each-other, etc.
Furthermore, many of them are quite self-directed and stubborn. They get some idea in their head about something to try, and then want to carry the plan through and can get extremely frustrated if thwarted (whether by the plan's intrinsic problems, or by interference from other people). Sometimes the plans are problematic (e.g. «I wonder if I can get adult attention if I take every toy my younger sibling wants to play with, until they start sobbing»), but often the goals/plans are perfectly innocuous, but adults step in and block action for poor reasons or no reason at all, or because the adult wants the kid to be focusing on something different. After being thwarted at a few different projects in a row, kids can become very agitated and defiant, or even melt down completely.
Once the kid has accomplished their immediate goal or given up on it, they sometimes instantly switch to some completely different idea, and don't want to switch back.
It is incredibly difficult to convince a big group of young kids (and especially boys it seems, though many girls are also very active) to sit still and focus on some specific close work without conflicts, especially when they have pent up energy. Trying to force them ends up creating a confrontation, and plenty of kids are looking around for ways to push boundaries and get attention. This is entirely normal age-appropriate behavior, and should not be pathologized. Letting the kids choose their own activities and decide when to switch from one to another goes a long way toward avoiding unnecessary bad feelings.