didn't drop out of highschool myself, but I think a third or half of my group of friends dropped out in the 4th or 5th year. most of them didn't drop out entirely though, just went to a different school. I don't exactly know how highschool in other countries is organized, in my country there are 3 different kinds/levels of highschool, only with the 'highest'/most theoretical kind you are accepted to university, wich is the kind I went to, most of my friends dropped out to the middle-level. a few of them dropped out entirely though, but they all first dropped to a lower level of highschool before dropping out entirely.
my observation was that motivation was the problem. I myself had already decided what I wanted to study after highschool while I was in 4th grade, and since it was a universitystudy, I knew I had to finish that highschool, so I often put in a little more effort during the last few weeks of the year to work away just enough insufficient marks so I could pass the year.
but some of my friends didn't know what they wanted yet, so they didn't have a good reason to put in that extra bit of effort. one friend I'm still in contact with eventually did decide what he wanted, and is now studying at a university(he did drop out to the lower level of highschool, so he had to do entrance exams to be allowed to university), but there were also 2 guys that dropped out entirely. both I haven't spoken to for a while. one of them, after dropping to a lower level of highschool, kept showing up late or not at all for exams, then dropped to anotrher lower level of education, then dropped out from that too, no idea what he did after that.
the other guy stayed at the highest level of highschool, but at a different school(technical thing, he couldn't pass the year at my school due to his grades for latin/greek, but he could still pass if he went to another school without latin/greek). but his motivation was low, and he was also working at his father's company to make some extra money. that job went so well(mostly bartending I think), that it got even harder for him to bring up the motivation for school, since he was already making plenty of money without education. eventually he decided to quit for the year, but start again the next year, and started working more. don't know how that ended up, but I expect him to have finished some sort of highschool education by now.
among my wider group of friends(not just who went to the same school) there were a few more, and in all cases I saw the same underlying pattern: still figuring out what they wanted, and therefor no motivation to finish education. some of them did finish some kind of highschool education, but couldn't decide after that, (again, don't know how it is in other countries, but here you have to go to school untill you're 18 or have a diploma, or else your parents get in trouble, and most people have finished highschool by the time they're 18). I only had one friend who dropped out entirely without plans of starting again, don't really know why but I think he mostly just didn't like school, and was rather hanging out in the park smoking weed. he got plenty of money from his parents to buy weed. I think he was just mostly living in the moment, not really thinking about what the future would bring, or maybe he was but just never mentioned it.