Cortina that's what the SAT (standardised aptitude test for reasoning) is, in theory. The SAT IIs are an attempt to have subject specific tests which everyone applying to the university takes, which we already have here in the UK (A-levels).
In practice, some kids are drilled for it and do hundreds of practice tests which you can buy online, do courses at school or with Kaplan or the Princeton Review or one of the many other for-profit test-prep outfits. I'm speaking from intimate personal experience here...god, the hours of practicing timings for the reading comprehension section, the test techniques (skim read paragraph, read the questions, go back and read again and underline points relevant to the questions, if you can't answer a question make a guess and move on after precisely 40 seconds etc etc etc)
Finally the SATs are expensive and you pay for each sitting in cold hard cash - so some kids can only afford one shot, and other kids can afford to take them more than once to improve their score.
Again - there's absolutely no way you can winnow out the kids that have parents willing to go to any lengths to give them a leg up. What we as a society should be doing is making sure that even kids from disadvantaged backgrounds are getting a great education at school and not trying to massage the criteria to make it appear that a student getting AAB is the same as one getting BBC because they're doing better relative to their peers at their school.