Lambertville is mentioned in the dialogue in the car at the beginning. The word "Lambertville" is also written on the wall behind the Dead Girl. Also, the place (especially the outside, with the red bricks) is recognizable in some of the pictures on urban exploration sites.
The Lambertville High School was closed and left in abandonment in 1959 and gutted by a fire in 1992. Urban exploration is considered dangerous for structural reasons, and is also illegal since the place itself is owned by a private company, and the premise is regularly patrolled by the police. This however hasn't stopped the urban explorators, or the graffiti writers.
The school is rumored to be haunted, but the stories of violent deaths connected to it are legends.
The rumors about the place and the fact that it was destroyed by fire are probably what led the main trio to identify it with the Land of Ashen Waste indicated by the mysterious postcard, prompting the trip with Jesse and setting in motion the events in Cops Checked, No Body. They've since abandoned this theory - they now believe the postcard wanted to lead them somewhere else.