First, there is no US curriculum. States follow different programs. Independent schools such as SSM (Faribault) are entirely independent.
UK curriculum works towards two public examinations - GCSE and A Level. US doesn’t.
UK university admission depends almost entirely on A Level results. US university admission depends on a number of factors, including record of grades during high school years, student essays, teacher and counsellor recommendations, results of AP examinations and SAT or ACT test scores.