It is possible to graduate from Barnard without ever seeing someone on drugs, taking drugs, or talking about taking drugs. They're just something that people who go to Barnard don't mess with, usually because most of the students were never exposed to that sort of scene. New York City and the area right outside of Barnard are crawling with drugs, and most students know that should they ever want to try any sort of drug, they will be able to find it with relative ease. The most common drugs on campus are either drugs that enhance a party, like ecstasy; drugs that help reduce stress, like marijuana, alcohol, or nicotine; or prescription drugs like Adderall to help students stay awake to study.
It's not like Barnard really has to do anything to keep drugs off campus. The school is really kind of made up of kids who want to be in school working hard, as opposed to kids whose parents made them go to college. Most of the girls at Barnard have a very specific agenda and won't be deterred by fleeting things like drugs. The only type of drugs that anyone at Barnard might use would be awareness-enhancers to help with staying awake or studying, but that isn't even very prevalent, either. All things considered, there isn't really a drug scene at Barnard. If you are into drugs, you'll be able to find them, but you probably won't want to because no one else will be doing them, and there's so much else going on.