My parents were cool about it. When we were in high school, they let us know that they werent stupid, and that they knew when we went to parties we were probably going to drink. They said they would respect our decision to do so if we respected their rule to do it responsibly.
As for drugs, they werent idiots about it as well. My parents admitted that they "experimented" in college, and were fair about talking about the no-bullshit side of drug use. My mom said recently that she would rather we smoke a joint every 3 or 4 months then to get drunk every weekend.