A Brooklyn high school student with a gun hidden in his backpack was apprehended when the weapon set off metal detectors as the teen tried to re-enter the building after dismissal Tuesday, averting a potential tragedy, cops said. The 16-year-old student at Samuel J. Tilden High School in Flatbush was disarmed around 4:12 p.m. after he tried to get back inside the building when school had already let out for the day, according to police. Authorities found the 9mm pistol in the teen’s backpack, cops said. The gun was loaded, according to a police source; NYPD officials couldn’t immediately confirm that detail. The student, whose identity is not being released because he’s a minor, was taken into police custody, and an investigation is underway, cops said.