I can see this getting really confusing for Americans, let alone the rest of the world.

What happens if you move from one school in one state that requires little, to a school in a different state that requires the 4 years of Science, Maths and English, and you hadn't done them?

Would you be kept behind, or just find a different school?

Over here EVERY high school does Sciences, Maths and English for 5 years (after 3 years Science and English get broken down into Chemistry, Physics and Biology, and English Language and Literature). I suppose PSHE coutns for Social Studies.