Barely There – Sometimes there's a hopping party at Bryn Mawr, maybe 3x a year, but it's not that great. Public Safety makes sure the servers don't give anyone more than 4 drinks at the party, and keep the number of people in the building at capacity. It's for our own safety but it's still not fun.
There are some parties at Haverford and Swarthmore that you can go to via the Blue Bus, but they're generally small. Your best bet is to go into the city to find a party at Upenn or another Philly college. That's what most underage students do if they want to leave tri-co, but you have to take the train if you don't have a place to stay over night and the last train is around 2am.