For much of the school year, seven students at McKinley High School in Northeast Washington often arrived late and left early. The classes they attended were sometimes disrupted by the beeping of the electronic pagers they wore with their fashionable sweat outfits and expensive tennis shoes.

This week they were arrested and charged with selling the potent and popular drug PCP to their classmates and friends. Police say they carried tinfoil packs of marijuana sprayed with PCP in their pockets and gym bags and did business openly in hallways and the school parking lot.

These teen-agers are a part of a growing and sophisticated network of youngsters who have become businessmen in the lucrative field of illegal drug sales, which can bring a seller $200 a day, according to police.

The arrest yesterday of a 19-year-old McKinley student brings to 12 the number of arrests for drug sales and possession that have occurred in the city public schools since January. There were seven arrests in this category all last year.

It was the beepers that made these average-looking students stand out from their contemporaries, said police who made the arrests.

The small device, favored by legitimate businesses as a way to get messages, has become an integral part of the distribution system within the PCP drug network in the schools. The sound of the beep indicated it was time to pick up drugs or make a delivery to a customer, police allege.

"The beepers are like a badge to the kids," said drug counselor Edgar Johnson, who works with about 35 youngsters at the nonprofit community-based Concerned Citizens on Alcohol and Drug Abuse Inc. in Southeast. "It says they are a big boy, a lieutenant in an organization. The message to the other kids is you got to be crazy to mess with him because he has an organization behind him," he said.

D.C. School Superintendent Floretta D. McKenzie said that she plans to ban beepers from schools within two weeks.

"We don't want the schools to become a drug marketplace," she said yesterday. "We must let our young people know that illegal activities will be punished severely."

Washington has long had a well-established illegal network of drug sellers catering to adults in heroin and cocaine and prescription drugs. But according to police, youngsters have taken over the PCP sales both on the streets and in the schools because of the drug's great popularity among students.

City officials have said the use of PCP in the city is epidemic. This week's arrests of the seven McKinley students and another at Cardozo High School in Northwest signal what police call a long overdue investigation into the drug market within city schools.

"There has been a rapid escalation of the sale of PCP and cocaine in the schools," said D.C. police Detective Bill Larman, head of the four-member "school squad" that was responsible for the eight arrests citywide this week. "They are more aggressive about selling PCP because there is more money to be made than in selling marijuana. The kids can sell PCP at $15 a crack any day of the week in school."

Police report a tenfold increase in PCP arrests for all ages in the past four years. The number of PCP drug charges filed against juveniles rose from 353 in 1984 to 545 in 1985, according to police figures.

Selling PCP means lots of money and little risk for pushers under 18, police said. Young sellers believe, and police concur, that they will receive little or no punishment if convicted of drug sales because of their age.

But these young businessmen suddenly have money to spend on items for themselves that their families cannot afford.. Some buy designer clothes, gold jewelry and sometimes expensive sports cars while others buy more drugs to sell or to use themselves.

A 16-year-old, who asked that his name not be used, said he actively sold the drug for the last two years until arrested recently on a charge unrelated to drugs.

During the summer he could make $800 a day by working all night with an older cousin, he said in a recent interview. But his gross dropped when he returned to Hart Junior High School in Southeast in September.

"I sell it during school time or when we go out for lunch," he said. "One of my cousins would come over to the house and get the money [every night], and I'd get paid at the end of the week. I'd get about $500."

After two years of working for his cousin, the young man decided to go into business for himself around Christmas.

But then he discovered the problems of being an employer.

"I had eveybody selling for me," he said. "That really messed it up for me."

Of one employe, he said, "I gave him 12 bags of lovely PCP and he was supposed to bring me back $110, but when I came around his house, he had $10 and five bags."

"I said, 'Where is my lovely at?' and he didn't say nothing," the teenager recalled. "So I said take these five and go sell them and bring me back all the money. But when he came back, he had nothing and he was high.

"That is when I whopped up on him," he said, swinging his right fist into the air, his face contorted in anger at the memory.

His arrest occurred shortly afterward, and he was required by his detention officer to keep a 9 p.m. curfew, he said.

"This lady had to call my house at 9 o'clock, to make sure I am in the house at 9," he said, pouting slightly. "I usually be out there about 12 or 1, a whole lot of the time. That is what slowed me up a whole lot because 9 is just when business just begins to move, especially on Friday and Saturday."

The young dealer finally went out of business because he said he just couldn't run his operation from home. He said he is now concerned that the $800 wardrobe he bought in September is wearing out.

"They are very clear about wanting the money," said Johnson. "They want to be able to buy those designer clothes that their mothers can't afford. For that age group, the image is very important. They often kid me about my $39 tennis shoes and brag that they spend $139 on their shoes."

Johnson said that from the window of his modest counseling room at 3119 Martin Luther King Jr. Ave. SE, he watches the students from Ballou High School come to the nearby carryouts to buy lunch.

"It's the big-shot sellers who get the new cars," he said. "They show up out front of here to sell to the students. I see them in their new Audis, Saabs and Datsuns with the paper plates. Most of them look barely old enough to have a driver's license."

McKenzie said she saw little chance of discouraging drug dealing in the schools until the situation improves on the streets.

"If the drugs are on the streets, they are in the schools," she said. "That is why we are cooperating fully with the police department. They have the schools under surveillance during the school day and at night."

McKenzie said that the eight students recently arrested have been suspended for 10 days, but that the school system is prohibited from expelling them.

"I am very much concerned about the distribution of drugs," she said. "When one kid gets another kid to try drugs, that says they don't care."