How well students do at any given medical school seems to be a pretty well hidden statistic. The National Board of Medical Examiners releases the performance of a medical school only to that school. Medical schools often use their performance on the USMLEs to evaluate their courses. Some schools have posted their USMLE score distributions (for example, The University of Florida Medical School and The Virginia School of Medicine).
Since performance on the MCAT correlates relatively well with performance on the USMLE (see MCAT predicts USMLE scores), schools with the highest admissions MCATs are likely also to have the highest USMLE scores.
You can find basic pass fail statistics at the NBME USMLE site
Here's a distribution of scores for all test takers:
Some bench mark numbers (from UC Irvine Residency Selection Handbook and the NBME USMLE site):
top internal medicine programs are looking for step 1 scores of 230 or better.
the average usmle step 1 score for applicants who matched in ophthalmology had a score of 224 (those who didn't match had an average score of 206).
Passing score (as of 1/16/04) is 182 according the NBME official USMLE site.
The average score is about 214 (estimate from the score distribution).
None of this is "official" or "gauranteed", but I do think its in the correct ballpark.