Most universities in America rely on the SAT, the Scholastic Aptitude Test, as to whether to accept a student or not. As there is no national testing like in Britain (A'level) or Germany (Abitur), the SAT largely determines university entry in the United States.
Now the National Foundation for Educational Research has piloted the SAT in Britain. About 1,300 pupils both from independent (private) and state schools sat the test, which includes English comprehension, maths and history and takes three hours. There were two interesting features resulting from the test:

There was broad correlation between good SAT results and good A'level results with students from independent schools. A good SAT result is a score of 1,200 points. Such a score would be necessary for a student to be accepted by Harvard.

30 pupils out of c. 600 from poorly performing state schools also achieved the magic 1,200 points and-what is surprising- their SAT results and their A'level results were enormously different. 4 out of those 30 achieved only the equivalent of 3 E-grades.