Lincoln High School initiated a lockdown shortly after noon when a tip was received about possible intruders, according to a spokesman for the city's public schools.
When a search failed to find any intruders, the lockdown was ended by 12:30.
Reports of fights in the parking lot or hallways are untrue, Fernando Gallard said.
The school is expected to close at its normal time, he said.
Abraham Lincoln High School is located at Rowland and Ryan Avenues in the Mayfair section of North Philadelphia.