So I can only speak for Denmark, and not even too much. High school is ages 14-18-ish, right? So that's mostly just school for us, as that ends when we're 16/17. The Danish system is: school 9 or 10 years, optional after-school 1 year, gymnasium/HF 2 or 3 years, university or trade school. After-school is a sort of boarding school for people who want either another year before they commit to the career choices implied in gymnasium/HF or who want to try living away from their parents without the commitment of actually moving out (it's oddly popular with people on the autism spectrum), or who need another year to get into HF (you have to be 18 or older). The 10th year of school is optional and not popularly chosen. I don't think you'd be too wrong to think of the last few years of school as an extended middle-school.
HF is usually geared towards adults who want a career change but need some more gymnasium level classes to get into their new education, but many teens also go there to get around the debauchery of gymnasium or just really want to streamline their education and only take the necessary classes to get into their higher level education of choice. I personally went to HF, so I got to be in a classroom of mostly adults who wanted to be there (I was the youngest, while the oldest was 30 years older than me), which the teachers were grateful for. HF can, depending on what classes you need and take, last anywhere from 1 to 3 years, half years included. There're almost no school made social arrangements.
As I said, I didn't attend the much more popular gymnasium, but my brother did and I visited some. Parties are a big part of the students' lives, as they can legally drink and are suddenly viewed as more adult, to the point where the schools try to throw some of these parties to maybe keep a lid on it (I'm not sure, I was very confused when the principal came to tell my tour group about how they would book a club for an annual party for the students). I don't think cliques are really a thing, and they certainly don't have a hierarchy, though there's probably some tendency for people to flock into somewhat similar social groups. Notably, my asocial brother became very well-known around his gymnasium for always wearing a beret, to the point that complete strangers greeted him by name in his third year. There are some more formal school dances in gymnasium aside from the parties, two, I believe.
In either case, most kids probably transport themselves to school via either bicycle or public transport, unless their parents find it convenient to give them a ride, only the richest of families would splurge on a car for the kid as they are a great expense. Sports are, at this stage, something kids play in their spare time for fun, maybe for junior clubs or hoping to get scouted, but the schools don't get more involved than having P.E., which might include a super informal tournament between classes or another nearby school.
AP might be something available to individual students based on the school and the student, and I wouldn't be surprised if that was the norm for some levels of certain classes, that they dipped into university territory.
Classes come in three... intensity levels, I suppose, A, B and C. C is the basic level and lasts generally a year or two, to be replaced with another C level in second year or upgraded to a B level. B lasts two or three years, and probably won't be replaced. A also lasts two or three years and might dip into college level in the last year. Danish on A level is mandatory. A second foreign language (besides English) is mandatory in gymnasium but not HF, it's usually German although both Spanish and French are popular options (one gymnasium offered Japanese).
In gymnasium and optionally in HF classes are sorted into packages, or lines, from which students choose, with only a few subjects undetermined for free choosing (such as second foreign language if the school has more than German). They can generally switch line in the first six months if they find it doesn't work out for them. An option only available in HF is "single subject" HF where you can pick and choose your classes and subjects, although you need a minimum of 23 hours of school a week to get SU (allowance from the government, only available to 18+).
I hope this wasn't an incoherent ramble, and that it made some sense you could use. It must have been weird to read "gymnasium" so many times and not have it mean "people exercising".