Look at this incompetence of Virginia removing accelerated math classes before 11th. Remember that American high school diploma is merely slightly harder than GCSEs in UK, per subject while GCSE is taken at 15-16 in UK. So those in UK are basically completing regular American high school diploma two years earlier.

AP is between AS level and A level in the UK. To put that in perspective 17-18 year olds sit AS levels and 18-19 year olds sit A-levels. You can see that US education needs to catch up.