As about 5O horrified students scattered for safety, a young man wearing a hood and surgical mask yesterday shot and seriously injured the business manager of St. John's College High School in northwest Washington during a robbery attempt in the school cafeteria, District police said.

As he fled from the lunchroom, the man shot and wounded a 15-year-old student who was standing outside near the cafeteria doors.

Police briefly questioned two persons yesterday about the incident but made no arrests related to the shooting at St. John's, a Roman Catholic military academy at 2607 Military Rd. NW.

The business manager, William J. McGregor, of 10220 Hatherleigh Dr., Bethesda, was in critical condition late yesterday at the Washington Hospital Center, a spokesman said, McGregor was shot four times as he struggled with the gunman on the lunchroom floor.

The student, Dennis W. Giuliani, who police said lives at 9916 Derbyshire La., Bethesda, was shot once in the upper right chest. He was in serious condition yesterday at Sibley Hospital, a spokesman there saidid.

No one else was injured police said. At the time of the shooting, shortly after 1 p.m., in addition to students and cafeteria workers, a small group of rummage sale that was scheduled for today, police said.

According to police, the man entered the cafeteria unnoticed and joined a line of students who were purchasing food. The man selected a carton of milk and a ham sandwich but when he got too the cash register, he displayed a gun, police said. When she saw the gun, the attendant at the register slammed the cash drawer shut and screamed, police said, whereupon. McGregor came forward and began to struggle with the man.

As the two men fought, shots rang out and the students and others in the lunchroom ran for nearby doors and corridors or fell to the floor, according to accounts by police and witnesses.

During the scuffle, McGregor was shot in the face, right leg and arm and below his right ear, police said.

After McGregor was injured, the man went back to the cash register and then fled from the cafeteria, police said. They said it had not been determined as of late yesterday whether any money was taken during the incident.

As the man ran outside the building through a set of wooden double doors, he encountered Giuliani, and knocked him to the ground, police said. Police said it was unclear yesterday at what point Guiliani had been shot.

One student said yesterday that Giuliani ran outside the cafeteria when the incident began.

According to witnesses, the gunman then ran north on the school grounds towards the gymnasium.The school campus is bounded on the north and west by residential neighborhoods and on the east and south by Rock Creek Park.

Late yesterday, both U.S. Park Police and District police with police dogs searched thoughout the surrounding area without success for the gunman.

Lt. William Bridegum, of the second police district, said the initial investigation indicated that the man was armed with a small caliber handgun. Six shots were fired, Bridegum said, four hitting McGregor, one striking a wall in the cafeteria and the sixth wounding Giuliani.

Bridgegum indicated that police are considering the possibility that the gunman, described as being about 17 or 18 years old, may have been familiar with the school because he seemed to know its layout and rountine.

Police said the man's face was almost completely concealed by a large, fur trimmed hood and a surical mask. The man wore a blue jacket, blue jeans and tennis shoes, police and witnesses said.

School administrators could not be reached for comment about yesterday's incident. The robbery attempt took place close to the end of the school day and by early afternoon all the students had been dismissed and the school doors locked and chained shut. More than 900 students attend the military school in grades seven through 12.

In December, 1974, two men, one armed with a gun, robbed the St. John's business office and escaped with money taken from a safe.