Huh, I'd liken school in those grades to a series of meetings basically all day, that are mostly presentations, every day of the week, with a lot of restrictions and harsh conduct & expectations from the people leading the meetings, which'd never fly in an office. Often with terrible lighting and long stretches without seeing the outdoors, even though a window. And crazy-early start times that may have you not seeing the sun until 3PM or so, for months. And, especially toward the end, a couple more hours of work at home every day. Mostly of math problems.
Also, all that, plus you're not getting paid for it.
If that were a job, I was characterize it as a boring job, not a stressful one.
I'm sure experiences differ, but mine was that school was trivially easy and inconsequential, but sometimes time intensive.
I wouldn't consider a job where I have to go and listen to a boring presentation for 8 hours a day stressful. What is stressful is the rat race and making sure I can afford mortgage payments
OTOH, if you fail out of class, most of us wouldn't have become homeless, just placed in a remedial class. Whereas getting fired from your office job would land you out on the street if you're living paycheck to paycheck.
Only if you live somewhere without unemployment insurance (or you mis-manage your money).