George W. Hewlett High School pupils were allowed to re-enter the school on Friday afternoon after the discovery of a pressure cooker there prompted an evacuation, Nassau police said.

The school’s staff and pupils waited on the athletic track until the police, including investigators from the Bureau of Special Operations and the Arson/Bomb Squad, finished inspecting the campus, school officials and police said.

“The building was deemed safe,” a Nassau police spokesman said. Police did not say how long the evacuation lasted.

School officials reported finding the pressure cooker along with a backpack to the police at about 12:34 p.m., police said. The pressure cooker later was determined to have been used in a science project.

Some educators use pressure cookers to demonstrate principles of thermodynamics.

But pressure cookers also have been used in bombs; the kitchen tool was used in bombings at the Boston Marathon in 2013 and in Manhattan’s Chelsea district last year.

Friday was “Clean Out Your Locker Day” at the school, a police spokeswoman said earlier. It also was the last day of classes, according to the school’s website.

advertisement | advertise on newsday

School security video is being examined to see if it captured who placed the pressure cooker where it was found, according to News 12 Long Island.