A high school diploma is just an indication that a student has passed the minimum number of classes in subjects required by the state and the local high school district.
My DCs technically had to gain 43 credits in order to get their parchment. This requirement was broken down as follows:
8 credits in English (= 4 years, 2 semesters per year)
6 credits in Mathematics (= 3 years, 2 algebra, 2 geometry, 2 other)
4 credits in Science (= 2 years, 2 lab science credits necessary, plus 2 other)
4 credits in History (= 2 years, credits had to include world history and American history)
1 credit in Civics (= 1 semester, and students had to pass a constitution test)
1 credit World Languages (= MFL or Latin, 1 semester)
1 credit Fine/performing arts (1 semester)
1 credit Applied Arts (choose from Family/consumer science courses, Engineering and tech courses, Business ed courses, Computer applications; 1 semester)
1 credit Computer proficiency (1 semester; used to be called keyboarding; requires proficiency in microsoft office suite, and there was a minimum typing speed/accuracy expected)
1 credit Financial Literacy (either financial literacy course or AP econ satisfied this; 1 semester, but AP econ was a 2 semester commitment)
1 credit Health (taken in freshman PE class for one semester)
1 credit Drivers Ed (done during PE in sophomore year; proof of passing state test required if students failed this school driving course; alternative credit required if student did drivers ed outside of school course).
6 PE credits. You had to take PE every day for three years even if you played an extra curricular sport. Exemptions were granted under specific circumstances but students had to earn one alternative credit per released semester if given an exemption.
7 credits for Elective subjects.
That's the baseline in my local HS. A student could theoretically gain a 4.0 grade point average while taking these minimum requirements.
Kids who are heading for university generally take on far more courses and at a higher level than the number and level required for graduation. Because of weighted averages, kids taking honours level and AP level courses can end up with GPAs above 4.0.
Kids heading for university also normally do the SAT or ACT or both (though these standardised tests are not required any more by some universities) and I would suggest that this is actually a kind of national exam which is an equivalent of A levels, though obv not in a specific subject. (There used to be SAT subject tests too). There is also the honour of attaining National Merit Finalist status in the PSAT/NMSQT, a national standardised test administered to Juniors (year 11 of 12) which can result in university scholarship money. The top 0.5% of candidates nationwide are named National Merit Finalists.