The local British Consulate to where you are in the US should be able to help 'validate' your US education level. Otherwise consider this:
GCSEs at 15-16, graded A-E plus F (fail), and N (not gradable). Anything taken after 1995 and below a C means you are abysmally hopeless at the subject. Before 1995:
A* - brilliant (top 1% in the country for that exam)
A - very good
B - above average but not brilliant
C - average
D - below average but not poor
E - weak
F - failure
N (or U) - brain the size of a proton (at least for that subject)
Generally most kids with half a brain will take 9 GCSEs in the core subjects:
English Language
English Literature
Maths
History or Geography
Physics, Chemistry, Biology
1 European language, most commonly French, Spanish, or German
Above average kids will take up to 12. Anything more than 12 is a waste of time.
A levels: before 2000, most kids did 3. Since then, most kids are doing 3 full ones and 2 half ones (AS levels). You can do them in just about any subject. Graded similarly to GCSEs, except without the A*.
The important thing to realise is that
1. There are is no fixed mark above which a particular grade is applicable
2. A levels in their current format were introduced in 1991, and have been getting easier every year since.... errrr, sorry, I mean "grades have been improving" every year since.