The arrest of two sixth-grade boys Tuesday on charges of conspiring to rob their school cafeteria at gunpoint came about because the boys had discussed robbery plans with five other boys during recess the day before, according to the security chief of Prince George's County schools.

Security chief Peter Blauvelt said seven boys, all aged 11 and 12, at Columbia Park Elementary School in the Kent Village section of Landover, talked on the school's playground Monday about a plan to rob the cafeteria. Five of the boys "never planned on taking part," Blauvelt said, and one of those informed his mother that evening and she alerted the school the next morning.

Sixth grade teachers at the school of 400 students were advised by principal Robert Frazier to keep their eye out for weapons. Blauvelt said one teacher, acting on a suspicion, checked one student's book bag and found a .357 magnum handgun and a box of bullets. A school security counselor was called to the school and when he saw the gun, county police were called.

Blauvelt said that according to police, an elaborate robbery plan was concocted, involving an escape in a van to be driven by a male teacher who was to be taken hostage, and a hideout in a West Virginia cabin.

Blauvelt said the boy who allegedly brought the gun to school had taken it from his home. Both boys have been suspended from school and are being held at Boys Village, a county detention center.

They have been charged as juveniles with conspiracy to commit armed robbery.

Blauvelt said that students, particularly in high school, have brought weapons to school in the past, but "in 12 years, this is the first time that we have had something like this happen in an elementary school . . .With that type of ammunition .357 magnum ammunition contains an especially powerful powder charge , there's no telling what could have happened," he said.

Frazier, the principal, said yesterday that things had returned to normal and that Tuesday's incident had been handled quietly. "There were teachers in the building who didn't know what happened" until he told them late in the day, he said.