- ~8 hours in school per day, often in buildings with minimal natural lighting. During the Winter, this may mean almost no sunlight all day (no recess, like in elementary school). Wanna make people depressed and give them SAD that'll stick with them long after they're adults? Just do this to them for a few years.

- Intense workload. We consider it bad when a job takes more than 8 hours a day from you. Schools routinely take 10 or more (math classes were the main culprit, at least in my case). Hope you don't have any other plans... oh look, many kids do, so now they're in actual hell.

- To add to the above: super-strict and rapid turnaround expectations on work. You cannot put something off until tomorrow because you're feeling really bad today—it was only assigned today, sure, but it's due tomorrow. Some stuff had longer timelines, but many things were the due-within-24-hours sort (again, largely math's fault, at least in my case)

- Bizarre mind games where people tell you insane stuff like "enjoy this, these are the best years of your life" and "you think this is bad, just wait until you're in the real world! This is just trying to get you ready for the expectations of adult life!" Like, I've not only worked cushy high-paid white-collar jobs, and I've never worked in an environment remotely as bad as school grades 7-12, nor with those kinds of strict expectations, nor with such inhumane treatment. When a workplace is consistently close to as bad as school, it's news (Amazon warehouses).

- Jail-like conditions. Need to stretch? Need a quick stroll for your legs? Need to take a piss? Beg the boss and hope they're in a good mood. Granted, some workplaces are like this too (again: Amazon warehouses), but most of those at least give you a couple 15-minute breaks in addition to your lunch (passing periods don't count, they're typically only 5 minutes and you'll spend most of that grabbing your stuff and getting from A to B)

- Sitting in classes all day is about as bad and mentally/physically exhausting as sitting in meetings all day, for similar reasons. Ask most people how they feel after a full day of meetings. Expand that to a whole week. Expand that whole week to 6 damn years. Yikes.

All of that is purely about the schools themselves, setting aside their strong tendency to foster awful, abusive, bullying dynamics that students cannot escape, both among students and staff. Or problems with school start times and teen sleep patterns (shit, as an adult I've rarely needed to wake up at 6:50 for anything, and if I did and I hated it I'd at least have some realistic hope of finding a way to change that pretty quickly)

I think all that makes it survivable (and I mean that literally) are the Summers and multiple long holiday breaks. When school's in session, it is brutal like few other involuntary (or de facto involuntary) activities are.

[EDIT] OH! And crazy-high expectations of self-organization and perfectionism. Here in the "real world", honest mistakes are taken in stride and my schedule and work-tracking are much simpler, plus I have a ton of support on those things. Even college tends to be far more lenient on those things than high school.

Again, I had a pretty damn good school experience, as those go, and my school wasn't one of those high-pressure ones you hear about in SV or wherever, and it was still terrible in these ways.