Not just public schools. I went to private school and still woke up with high school nightmares for years as a college graduate.

I think the expectations are a part of it, but I also think it's the fact that it's an extremely stressful time period between expectations for one's future, high hormones, major social changes that happen as everyone's brain cooks at a different speed, and the constant fighting for increased independence.

Like, don't get me wrong I also didn't enjoy being in class, and I've had full on flashbacks to fights with my father over my grades in high school (also have a ptsd diagnosis), but imo the educational elements of high school weren't meaningfully worse than earlier years or college/adulthood. It was being a teenager that was particularly traumatic, and high school was just the location where most of the issues converged