I've talked with an acquaintance overseas before about the differences between school here in the States, and where they grew up (Argentina), and one big difference I noted was that they pretty much stayed with the same class year-round, with the teachers being the ones who would cycle around classrooms. (I notice that this is also the case in Japan, and likely in other countries too)
In America, at least where I grew up, Middle School (usually 6th grade to 8th, tho sometimes includes 9th, or excludes 6th) and High School (usually 9th to 12th) go the opposite route, which is sorta more similar to how University works: each individual student has a schedule that is independent to them, and Teachers are the ones who basically stay in the same room.
In my state, High School required you to have a certain amount of credits to graduate (which meant you could take online or summer classes to get extra credits and graduate early), but there were a couple of required credits. Each semester of a class gave half a credit, so one full year of that class was a full credit. At the bare minimum, my state required that you had 4 credits for each of the following subjects: English, Science, Math, and Social Studies. Additionally you required 2 Foreign language credits, 2 Physical Education credits, and after that you can have any other elective you wanted, as long as you reached the minimum. There's also some edge cases, where certain classes might count as an elective credit, or a Math credit, depending on whether or not you took a different class already.
Note that the number of required credits, and which classes may count as what type of credit, are likely to be different between states.
Also, the dates that summer/winter break starts or ends may be different from district to district, or even school to school—the exact number of "off days" in my State are determined by the School District, and where those off-days go are more-or-less up to the individual schools. (Although federal holidays and some other breaks, like spring break, are always the same) I remember getting some days knocked off Winter break, because the school was closed for too many days earlier that year due to a Hurricane.
Anyways, these are just my experiences, and based on schools in my area. The U.S. hates being organized and uniform, (speaking of, whether or not a school had a uniform was also based on an individual school's choice—some let you wear your own clothes, with minor restrictions, while others made sure you bought your uniform from the school itself) so I'd expect things to be different in other states.
