Four students were busted after they sneaked a loaded handgun into a Bronx high school yesterday, police said.

Two 15-year-old boys first opened up a side door to let a 14-year-old with the .22-caliber gun into Alfred E. Smith Career and Technical HS in Melrose shortly before 9:30 a.m., cops said.

The teen then passed off the German-made pistol to another student, 14, who placed the weapon in his backpack.

But when the students were told to go through a metal detector, the boy with the bag threw it down and fled.

Police arrested all four boys.