A frustrated Queens substitute teacher Wednesday threatened to shoot a classroom of middle schoolers, then voluntarily confessed what he’d said to school officials, police sources said.

Alejandro Rocha, 52, was teaching an art class at Intermediate School 5, the Walter Crowley School, in Maspeth, when he blew his top, sources said.

“If you won’t do as you’re told, I’m gonna shoot you — and yes, I have a gun,” he told the nine kids, all ages 11 or 12, sources said.