The PA State Police sent fifteen cops dressed as college students to a Haverford dorm party Thursday night, citing more than thirty students for underage drinking.

Drinking in the dorms is allowed for over-21s at Haverford, and the party was advertised on Facebook. But cops planned the raid after checking out the profiles of students who’d put themselves down as planning to attend and finding that many of them were underage.

Police showed up at the party 10:30, hoping to arrive as it was getting underway, but by the time they got there most of the alcohol was gone. They hung around for half an hour, observing, then announced themselves and started asking partiers for ID. They detained about forty students, and issued citations to 31 of those.

One interesting tidbit: The cops didn’t give the university a heads-up before crashing the party. Haverford’s president, Stephen G. Emerson, learned about the raid when a student called him after the police started asking for ID, and Emerson arrived on the scene himself about half an hour later.