District police arrested six students at Anacostia High School yesterday on charges of selling illegal drugs as part of an ongoing drug investigation in city high schools.

The arrest of three adult and three juvenile students followed a nine-month operation at the school at 16th and R streets SE conducted by a young police officer who posed as a student.

Another adult student at Anacostia High was arrested at his home on Thursday and charged with selling drugs, bringing to 19 the number of students arrested at four city high schools since January.

Police Detective Bill Larman, one of the officers overseeing the school drug investigation, said that the undercover officer had no problem purchasing the illegal drugs.

"The officer started school on Day One, just like all the other students, and made his first buy within 48 hours of starting school," Larman said.

Two of the four adults arrested were indicted on Tuesday by a federal grand jury. According to court records, Eddie Bell, 19, who was arrested at his home Thursday at 2304 Green St. SE, was indicted on one count each of selling PCP and marijuana. Xavier Mack, 20, of 1275 Brentwood Rd. NE, was indicted on two counts each of selling marijuana and PCP.

According to court records, Mack had a previous conviction for possession of PCP in 1984 and was on probation at the time of his arrest. He also had been arrested Tuesday on a separate charge of selling PCP. Mack was held yesterday without bond.

The two other adults were charged with selling PCP and selling a drug within 1,000 feet of a school, according to police. U.S. Magistrate Jean Dwyer released them on their own recognizance, but ordered them not to go "within a block" of Anacostia High School unless she receives a written note from the principal authorizing their return.

The two were identified by police as Philbert Pierre, 20, 1845 23rd St. SE, and Christopher Banks, 20, 2907 M St. SE.

Court records show that Pierre is a native of Trinidad and had to surrender his passport. Banks gave the court his home address as 4908 Dublin Rd., Camp Springs, but said that he stayed at the M Street address with a friend to attend school.

The three juveniles, aged 15, 16 and 17, were not identified by authorities and it could not be learned whether they were held.

The arrests were made quietly during morning classes, according to Larman.

"We asked school personnel to escort the students from their classes and bring them to an empty room in the school," said Larman. "We arrested them at that time and took them from the school by the back door."

Larman said the students appeared surprised by the arrests.

"We told one kid that his arrest came from a sale he had made in November," Larman said. "He seemed to think it was unfair because it happened so long ago."

As the police patrol wagon drove away from the three-story school building about noon yesterday, some students said they knew nothing of the arrests but knew that drugs were sold in the school.

Arthur Kirkley, 18, said he knew that both PCP and cocaine were sold in the school, but that he had never seen a transaction.

"I think it is a good idea to have undercover cops in the classroom," he said. "They may prevent students from selling and buying drugs."