I don't have any stats to back this up, but one of my friends who is a doc said that at her school (Northwestern) only a few people ever dropped out/were kicked out, etc. When I started expressing an interest in med school over grad school she was really adamant on the point that once you are in med school they try their hardest to keep you in med school and will offer you lots of help to stay afloat, after all, you are in so much debt after even just a year or two, that it would really suck not to finish and have all those loans.
Overall, I think the attrition rates at med schools are fairly low, definitely under 10% I'd say as a best guess.
In the Caribbean, however, attrition is a huge problem. I've heard of classes of 400 dropping to 200 after second year.