In the fee paying sector we have kindergarten, prep for 4 to 13 year old, then senior schools up to 18. We used to then have technical colleges (trade based skills), polytechnics (somewhere between technical colleges and Universities) and Universities, but then polytechnics became universities. Most degrees are three year courses, but differ from your Uni degrees because students specialise from day one. Some degrees, such as law, medicine, dentistry are longer. Students can remain at University for Masters or PhDs or they can complete them alongside a job.
Although the majority of children take exams at 15/16 and 17/18 and they are called GCSEs and A levels, there are a number of exam boards and they have varying standards. Clever teachers shop around for their weaker and stronger students, thus enabling them to achieve the best grades. Not all ‘A’s’ are equal. At 18 the majority of schools sit A levels and the various grades are worth points, and you need a certain number of points to read particular subjects at University. Many of the elite public schools, believe that A levels are too easy and restrictive and so they choose to prepare pupils for IBs or Cambridge Pre Us.
Whether a child sits A levels, IBs or Pre Us, the results are published in the summer (July/August) and until those results are out, young people don’t know whether they have good enough grades to enter further education.
I think that it quite different from the US system, isn’t it?
VERY different once you get to the point of GCSE's. Up until that point it's not hugely different, except that in most places in the US mandatory schooling is not required until age 6, in what we call First Grade, although most parents do voluntarily enroll children by at least age 5. Preschool (ages 3-4) and Kindergarten (age 5) are always available, though whether publicly-funded schools offer them sometimes varies. All taxpayer-funded schools here up through Secondary studies are locally managed and funded, and so they vary quite a bit in organization and curriculum. The standard is at least 12 formal years of elementary and secondary schooling, and each state sets minimum standards for the number of years one must study each core subject. For instance, my state requires 4 years of some kind of mathematics, 4 years of English language studies (literature, writing, etc.), 2 years of a foreign language, 2 years of social studies, 1 year of some kind of art or music, 1 year of American government, 1 year of US history, and the rest are flexible. Each year of study in each subject is worth 1 credit if you complete the assignments, and you need a minimum of 24 high school credits to earn a diploma.
The US has no leaving exams. You finish secondary school if you satisfactorily complete 4 years of high school, in whatever curriculum your local school offers (we also have no national curriculum, so that varies widely), and what you receive for that is a High School Diploma. (It is also possible in most places to leave school at 16, called "dropping out." It's also possible for adults of at least that age to take an exam called the GED, which also serves as a certificate of equivalent completion, though it is considered inferior to an actual Diploma, both by employers and by schools of higher education.)
The ACT and SAT are university entrance examinations; only needed for admission to academic study courses leading to university degrees. (Students intending to attend trade schools do not have to take them.) Students choose whether they want to take either one or both; they are administered by private companies and you pay to take them. You can take them many times in order to improve your score, but there is a hefty fee to sit these exams, so most students take them no more than 3-4 times, and many only once. Once you have a score you can live with (usually around November of your final year of high school, but often earlier than that), you submit admission applications to whatever universities you wish to apply to, and you normally pay each one an application fee for that.
There is no system of University "places" such as is managed by the UCAS. Different universities have different admission standards, but the process is more subjective here; average students who have desirable demographics and a compelling personal story might even get into Harvard if Harvard thinks that student will "bring something" to their student body. (An example would be David Hogg, the young man who led the public protests after the school shooting in Parkland, FL. His grades were OK, but he wasn't top of his class. It was his activism and public profile that got him in. Kids who are really star students often are offered admission to many top universities--even 10 or more.) Most universities will send an admission decision by March of final year, and usually require that students accept or decline by May 1st of their final year of high school. Most families end up making a final decision based on financials; if you have multiple choices, it may well come down to how much attending each would cost, and how much scholarship money each school may offer. In every state there are universities which have essentially open admission: as long as you have a HS diploma or a GED passing score, you can enroll (though of course you must pass your college courses to stay enrolled.)
The expression "to read a particular subject at University" makes no sense in an American context. We say "major in" a particular subject, but that isn't set in stone before you start there. Once admitted to a university, a student can declare whatever major they choose, but at most universities, there is a performance threshhold that must be met by the end of the second year in order to be formally admitted to the department that administers that degree program. American university students take a broad range of coursework for at least the first year and usually two, and only really begin to specialize starting in the 3rd year of their undergraduate program. Most undergraduate programs are designed to be completed in 4 years, but it's very common for students to take more time than that to finish; often closer to five, especially if they change majors somewhere along the way. (Personally, I started as a Secondary Education major, but in those days there was a requirement to pass a hearing acuity test at the end of the second year to be admitted for the degree. My hearing was not good enough, so I ended up getting a degree in History instead.)