A 17-year-old student was busted with a firearm at a Harlem charter school Tuesday afternoon, cops said.

The student was at Harlem Prep Charter School at 240 E. 123rd St. around 12:30 p.m. when the gun was found.

“An alert security guard noticed an altercation between some students in a stairwell,” a school rep said. “The guard stepped in, asked what was happening. One of the students said that he had a gun in his backpack. 

“We immediately called the police and locked down the school until the police finished their investigation and gave the all clear.

“We’re thankful to the police for their quick response and glad that the incident was contained.” 

Cops confiscated the gun. It wasn’t clear if the gun was loaded, a police spokeswoman said.