Penn weekends are three days long -- Thursday night is actually a bigger party night than Friday night. Fraternities have a large presence on campus, and a good portion of people do go out to party on the weekends.
With that said, people know when to go out and when to chill. It is relatively rare that people go out when there is something else to do (i.e. schoolwork). Also there is a large presence of non-drinkers on campus. Within your residence hall, there is bound to be a good portion of people who would rather chill than go out on weekends.
There are so many groups on campus that you can busy yourself with them and not feel like a loser for not going out.
If you are looking for a purely academic environment, however, Penn is not for you. The school is EXTREMELY pre-professional, and that includes sociability. Let's be real now, unless you are going to be working in a lab your whole life, you need to have some people skills, and Penn embraces being social.
There really is a good balance between coursework and partywork.
