Five students were arrested Wednesday after a handgun and drugs were found at Monsignor Bonner & Archbishop Prendergast Catholic High School in Upper Darby, police said.

The school was placed on lockdown for about two and a half hours while police searched for the gun, said Police Superintendent Michael Chitwood.

Around 11 a.m., a student saw a boy apparently selling marijuana to two girls at a trolley stop at Garrett Road and Lansdowne Avenue.

At some point, the boy pulled a gun and threatened the girls, Chitwood said. The three teens then returned to the high school.

The student witness informed the principal, who called police and the school was placed on lockdown.

Police and a drug-sniffing dog discovered marijuana in two lockers, and two male students assigned to those lockers were arrested, Chitwood said.

Separately, the male accused of having the gun was identified and was found to be carrying marijuana packaged for sale, Chitwood said. He also was arrested.

Police then were led to another boy who was allegedly carrying bullets in his backpack, and he was arrested.

Finally, the gun - a 9mm semiautomatic - was found in a girl's locker and she was arrested.

Chitwood called the student who reported seeing the gun a "hero," adding, "if they don't come forward, we don't find the gun in the school."