I could say a lot about the quality of education: the lack of a challenge, the boringness of the one-size-fits-all curriculum, etc. But boredom isn't that big of a deal. I made up for that by studying what I wanted to outside of school, instead of doing homework. Even with the vindictive teachers that made homework 30% of the final grade, it was still cakewalk to pass their classes without doing a single assignment.
The real root of the reason school was a living hell for me was the bullying. We all like to tell kids that "the teachers will help you", but they either don't care or feel they can't do anything. Either way, the net effect is they are completely useless. We tell kids, "it builds character!", but that's nonsense. I don't care if it is the secret to my success, I'd rather be average today than to have experienced a tormented childhood. Those were formative years, and those scars run deep.
The worst mistake I ever made was deciding I wouldn't be a part of that Lord of the Flies bullshit. I kept thinking it'd get better as we got older, but it only got much worse. I never stuck up for myself, never learned how to fight, never worked out to become stronger to defend myself. And that's easily my biggest source of regret. Every day my mind recalls terrible memories from those days, and I find myself constantly thinking, "if only I had fought back here or done this there ..."
Bullying mostly targets the gifted, but it would help everyone if we took it seriously, instead of just saying, "well I went through it and lived; so the next generation can too!"
Also, it's bordering on a different topic, but bullying very much extends to the home as well. Abusive parents, siblings, etc. Some kids have literally no escape from it for 18+ years.