D.C. police arrested a 15-year-old youth yesterday and charged him with carrying a loaded semiautomatic weapon on the grounds of a Southeast elementary school.

The youth was apprehended by an officer on patrol about 11:30 a.m.

The youth was walking outside Nalle Elementary, which is in the Marshall Heights neighborhood.

Police said the officer, Tyrone Scott, noticed a large bulge in the youth's jacket and stopped him.

Police said the youth was carrying a loaded Tec-9, a 9mm semiautomatic. No shots were fired and no one was injured.

Police said the youth was not a student at the school.

It was not clear why he had the gun, police said. The youth has been charged with violating the National Firearms Act, which is a felony.

Nalle Elementary, on 50th and C streets SE, sits in one of the most violent sections of the city, yet has been largely free of trouble. It has an education partnership with Howard University and a strong PTA. But many of its students live nearby in housing complexes where drugs and guns have been plentiful.

The safety of the city's students has been a hotly debated issue in recent years, since several students have been wounded by gunfire on sidewalks outside their schools.

The school board has not installed metal detectors, but has hired dozens of additional security aides. Last year, the board spent about $1 million hiring 60 aides for its elementary schools.

School board President Nate Bush (Ward 7) said yesterday he will ask Superintendent Andrew E. Jenkins to review security procedures at Nalle, and at the city's other elementary schools.

"I'm obviously very concerned about this incident," Bush said. "It serves as a reminder of how attentive we have to be to make our buildings safe."