Police say the threat was received by the 911 call center around 5:50 a.m. on Thursday.
Officers are currently on scene and working closely with school officials.
"Our department, with assistance from K-9 units and a drone team, has completed a search of the building and determined it to be clear," police said just after 11 a.m.
The Tredyffrin/Easttown School District had delayed the start of classes until they got the all clear.
"Our department will continue to investigate this incident and take all appropriate action moving forward," police said.