LATROBE — State police say they’re investigating an apparent senior prank in which the tires on 47 school buses were deflated, delaying classes in one Western Pennsylvania school district.

Trooper Steve Limani tells a Pittsburgh newspaper that someone jammed pen caps into the valve stems of the tires to deflate the buses owned by First Student on Thursday. The buses serve the Latrobe School District about 40 miles east of Pittsburgh.

Police haven’t named any suspects, but believe it was a prank because the words “Seniors 2011” were spelled out using duct tape on the side of one bus.

The district says officials in nearby Unity Township sent public works trucks with air compressors that enabled the tires to be refilled. The buses got students to school by 9:15 a.m., about two hours later than normal.