I recently posted about the desirability of attrition in colleges due to substandard performance, expressing the view that low graduation rates are a good, not bad thing, and that they lend some credibility to an expensive piece of paper.
Competitive and highly-selective graduate schools, however, probably should have lower attrition rates as their standards for entry are so relatively high.
Here is some info on Medical School Graduation and Attrition Rates
Here's Law School Rankings by 1L Attrition Rates