A 16-year-old boy allegedly left a loaded ghost gun stashed in a bookbag outside his Brooklyn school on Thursday — possibly to avoid detection, an NYPD spokeswoman said.

The teen’s girlfriend, also a 16-year-old student, warned Street Academy High School officials that her boyfriend was planning to bring the weapon to school, cops said.

Authorities were waiting at 832 Marcy Avenue when the school day started — but the boy wasn’t in possession of a weapon.

Cops then searched the area outside the school, which has scanners to detect firearms, and found the ghost gun in a bookbag stashed in bushes across the street around 11 a.m., the spokeswoman said.

The gun was loaded with seven bullets.

The 16-year-old boy was being questioned by cops.