Well you have your freshman, sophomores, juniors, and seniors. Basically, everyone has to take certain classes like Math, English, Science, but you can take "Honors/Gifted" Freshman and Sophomore year at my school. Then Junior and Senior year you can take AP Classes which are college courses that you can get credit for if you get a high enough score on the AP exams. (1-2 is no credit. 3-5 is credit).
Our grades are done by GPA 1-5. A 4.0 is straight A's. Most colleges want students with a 3.0 or higher. The higher your GPA, the easier it is to get into college. There are TONS of colleges in the US! But you also have to take the ACT or SATs to get into them and have to have certain scores to get in. Different colleges have different score and GPA requirements. The better school, the higher the price is to attend. If you have really good grades and test scores, then you can get an academic scholarship to certain schools.
There is seriously so much, and idk if I just made this confusing or not but let me know if I can go into detail and explain anything haha.