It happened after police converged on the unit block of East Forest View Road near the school in Parkside, Pa.
The reason for the police presence was not disclosed.
As officers arrived, school officials, acting on the advice of police, decided to evacuate the school.
About 250 students and staff were bused from Parkside to Coebourn Elementary School in Brookhaven.
Parents were notified that the students could be picked up there any time by end of the school day.
According to a letter to parents on the school's website from Superintendent of Schools George Steinhoff, "Students and staff were at all times safe, there was never a disturbance in the school whatsoever, and the transport of students to Coebourn went very well."
Police would not reveal any details about their activity.