A 12-year-old boy was busted with a handgun at a Brooklyn school Tuesday — on the second to last day of classes, cops said.

Somebody told a teacher at JHS 50 John D Wells School on South Third Street in Williamsburg that the boy had a 9mm Ruger, cops said. The teacher then took the boy to the office, where the gun was fished out of his backpack.

He was taken into custody around 11:30 a.m. by school safety officers. He hadn’t been charged as of early evening, and his name wasn’t released, cops said.

The gun was not loaded.