Lauri is just leaving, not knowing what is going on outside until she sees the patrol car. 

But there's no one inside the vehicle? Well, this is the chance to run away, surely the cops went to another house first, there are wild parties going on all over the campus. 

Nope, the patrol was sent right here by an anonymous call reporting to them that there was a party out of control at this address. But as soon as they learned that the host was "Sgt. Tera Lachance's son", they moved on to another dorm. The perks of having your mother in the police force!