The Upper West Side school that Barron Trump used to attend was evacuated and closed Monday when writing was found in a bathroom stall warning of an upcoming shooting there, law-enforcement sources said.

The alarming message at Columbia Grammar and Preparatory School said “shooting today at 12:45,” sources said. It was scrawled on the stall inside a girl’s bathroom at the West 93rd Street school.

The school closed about 11:30 a.m. as a precaution shortly after the writing was found and police were called, said Columbia’s head, William Donohue, to The Post.

“We had a security threat that we did not think was authentic, but given the times and what’s happening in the world, we handled it with caution,” he added. “Fortunately, we practice these things and felt pretty well-prepared.

“We expect everything to be back to normal tomorrow,” Donohue said.

The NYPD is investigating the incident.

Barron Trump, the son of President Trump and First Lady Melania Trump, used to attend the educational institution before moving to Washington, DC, with his parents.