A guidance counselor found a pistol underneath a desk at a Brooklyn high school Friday — one day after school safety officers caught two teens there with a firearm stashed in a backpack, according to school safety sources. A student’s tip led the guidance counselor to find six bullets and the gun bearing a sticker that read “marshmallow” at Metropolitan Diploma Plus High School on Rockaway Ave. near Linden Blvd. in Brownsville shortly before 1 p.m., according to sources. Sources say the frightening discovery is related to another chilling find slightly more than 24 hours earlier, when police caught two teens in possession of a loaded firearm at the same school. In the first incident, school safety agents tracked the gun to a backpack belonging to a 16-year-old boy around 12:23 p.m. Thursday, sources said. Before the gun was seized, the teenage boy handed off the backpack containing the weapon to a 16-year-old girl, who safety agents caught holding the bag, cops said. Both teens were arrested for second-degree criminal weapon possession and unlawful possession of a weapon inside a school, police said. No additional charges have been brought due to the firearm recovered Friday, cops said. The high school was placed into a “soft lockdown” — a state of readiness in which administrators and safety agents are alerted to a threat posing no immediate danger — on Thursday before responding police officers recovered the first gun, cops said. The Department of Education did not directly answer when asked if the school has a metal detector at its entrance. “We will be providing the school additional safety measures,” a DOE spokesperson said. Metropolitan Diploma Plus is a transfer school serving students who previously dropped out of school or have fallen behind in credits.