My high school and university experiences were decades ago, but here goes:
High school was terrible. Bullies, peer pressure, cliques, popular kids, etc. Granted, some of the popular kids were nice, but they weren't about to hang out with me. Also, some teachers were mean. I don't necessarily blame them, as they were dealing with teenagers every day. And although I (well, actually my father) could choose which stream I was in and which electives I'd take, there wasn't a lot of choice. So if I sucked at math, too bad, I had to suffer through. The only aspect of high school I enjoyed was the actual learning itself, and some teachers who encouraged us to really think about what we were learning.
I quit school the day I could do so legally, which was on my 16th birthday at the beginning of Grade 11.
At that time, most universities in my country had "adult student" policies, so I was able to eventually attend university. I went to university twice--for a few years in my early 20s, which was mostly fun and games and I didn't receive many credits; and then for two years in my late 20s, where I worked hard and completed my degree.
The first time, I had a blast, made friends, etc. Unlike in the US, the frat scene is not big here, and I didn't have to live in a dorm, so there was almost no peer pressure or being made to feel like an outsider. The professors and the learning were interesting. Other things were interesting too, like meeting students from all over the world. But I had no self-discipline and didn't do very well.
The second time, I was a single mother and desperate to receive my degree so I could get a decent job. It was just work work work. It wasn't fun, but I accomplished what I needed to. The learning itself was interesting. I didn't go to my graduation ceremony. I figured after all that expense and work, the least they could do was send me the damn certificate in the mail. And they did.
My grades were better in high school than in university. Maybe if I'd finished high school I would have been better prepared, but I didn't really understand the systems in university, e.g., what was expected in an essay, or how different disciplines approach knowledge differently.
If I were to go to university again, I would try to limit my "fun" to maybe one day a week, or to an hour a day. I'd still try to have friends and be sociable, but I'd concentrate more on finding friends who could support me in figuring out the system, what to do in order to pass, how to write an essay, etc. Unfortunately, the student groups that should have been useful for this (e.g., Anthropology Club) had other agendas.