A survey of medical school policies conducted by the American Medical Student Association gave the six schools an A grade for keeping pharmaceutical company representatives off campus and banning drug company gifts to students and doctors.

"An institution's ... policy is indicative of the ethical and practical foundation it can offer students," said Justin Sanders, a spokesman for the association, which is running the PharmFree campaign encouraging medical schools to eliminate the influence of pharmaceutical companies.

"Medical students want more and more to be 'PharmFree' and they are looking to their schools to move forward with them," Sanders said.

The six top-graded medical schools were at Stanford University, the University of California-Davis, the University of Michigan Medical School, the University of Pennsylvania, the University of Vermont and Yale University.

The group said it plans to conduct the survey each year.