It's practically 100%. Once you're in, they'll do everything in their power to graduate you (legitly). Every incentive they have points at a perfect graduation rate.
EDIT: In the US, anyway. Things are different in, say, Caribbean medical schools.
I read someone mention that French medical schools are easier to get into but the graduation rate is much more like engineering schools. About 1/2 to 2/3 of students drop out or fail. Said the result was French doctors were functionally better in general than US ones and happier.
Since I don't have any experience with French medical schools I don't know if that is true or still true.
I do have experience with US doctors and about half of them are just phoning it in every day.
EDIT: In the US, anyway. Things are different in, say, Caribbean medical schools.