Bingo, it's the trauma.

High school in particular was by far the most stressful sustained period of time in my life—I've had the odd day or week that might measure up, here and there, but never months in a row (at least you get Summers off). I've very quickly quit jobs that turned out to be a small fraction as stressful as high school was—they're vastly worse than a decent work environment, but still not as bad as high school. It's that bad. And I had a pretty easy high school experience! No intense bullying, no gang violence happening at/around school, none of that stuff.

9-10 hours a day of work (if you're doing all your homework, anyway), plus more if you actually want to be earning any money. Rushed minutes-long passing periods several times every single day. Having to ask to go take a shit if you didn't manage to fit that in to your few moments of semi-free time. About 6 hours per day comprising ~50-minute stretches that are about as draining as sitting in that many meetings of that length in a day (which is quite the fuck draining). Super-early mornings in most places, in the time of the life that's the very worst for that kind of thing, physiologically and psychologically. The sheer weirdness of spending almost all your time with a whole bunch of people about the same age as you.

The number of people you are around and interact with in a day, navigating their emotions and demeanors and all that, teachers and students and administrators, exceeds what I've ever seen at any job by a long shot. That you may at any moment be subject to behavior that would get someone arrested or at least fired from a job in the adult world, but you'll still have to be around the perpetrator for potentially years with no real way to leave yourself.

Immovable extremely short deadlines and unchangeable scope—hope you don't have the odd day where you just don't feel up to it, because the thing they assigned you today is due tomorrow, period. All your work literally thrown in the trash soon after you finish it. Near-zero tolerance for real life getting in the way of the work. Sick? LOL, no, there's not someone to take on a little extra work so you're not doubling up when you get back—it's all on you, have fun with the pile of inane tasks to complete while you still feel kinda shitty. Personal days? "Mental health" days? LOL no.

It's very, very bad. Strict rules and enforcement, weird and toxic social environment, tight and short deadlines completing assignments that serve no external purpose whatsoever covering things you don't care about (sure, that's kinda just how education has to be, at least to some extent, but that doesn't make it not-demoralizing).

I don't love the comparison to prison but it really is one of the closer analogs from the "real world", or maybe notoriously-bad careers/jobs (Amazon warehouses?) that employ people with few other options, though in my experience even most low-wage, low-skill jobs aren't anywhere near as bad as high school (but I've also not worked in the notoriously-bad ones).

My high school dreams (well, nightmares) finally mostly stopped... in my early 30s. Mostly. That's a hell of an effect. And again, my experience was surely in the top half, at least, as high school experiences go.