A 20-year-old former student was shot in the entrance of one D.C. high school and a 17-year-old student was stabbed inside another in separate incidents Thursday that drew attention to the problem of school security.

The 20-year-old was shot in the chest in the main entrance of Eastern High School during a quarrel over a female student. An 18-year-old, also a former student at Eastern, was arrested and charged.

In the second incident, a Cardozo High School student was stabbed in the back as the result of what police said was another quarrel.

Police said yesterday they have obtained a warrant for the arrest of a man in his 20s in the Cardozo stabbing.

Describing himself as "very, very upset and disturbed" about both incidents, School Superintendent Vincent E. Reed said school security is made difficult by fire regulations that forbid the locking of any school entrances and exits.

The superintendent said he has scheduled a meeting with Norman Richardson, the city's new acting fire chief, to discuss the possibility of closing some doors, particularly at schools such as Cardozo, which he said "has close to 80 exits."

Just before the shooting at Eastern, the victim, Anthony Armstrong, and his alleged assailant, had been quarreling outside the school at 17th and East Capitol streets.They ran inside and Armstrong was hit by a shotgun blast.

Armstrong of 1218 C St. NE was listed in fair condition last night at Georgetown University Hospital.

Samuel Daring of 675 K St. SE was arrested and charged with assault with intent to kill, police said.

In the Cardozo incident, the victim, Calvin Johnson, was stabbed in a hallway outside the cafeteria of the school at 13th and Clifton streets NE. h

Johnson of 3121 Sherman Ave. NW was listed in fair condition last night at Children's Hospital.

Third district police Capt. Richard Simmonds declined to identify the man for whom the warrant had been issued but said he was not believed to be a student.