A teenage boy was stopped by security when he tried to bring a realistic-looking BB gun into the school building Wednesday morning, cops said.

The 15-year-old student was walking through the metal detector at Erasmus Hall High School on Flatbush Avenue near Church Avenue around 8:45 a.m. when school safety officers discovered the gun and removed it, according to police.

Cops could not immediately confirm whether the teen is in custody or will be facing charges.