The low point of police cadet Tim P. Black's three-month investigation into drug use at Howard County high schools came only four days after the undercover operation started.

Earlier that week in February, police in neighboring Anne Arundel County arrested 33 high school students and 14 adults who had sold drugs to investigators posing as teen-age drug users. Black, posing as a Hammond High School junior, was confronted in the school parking lot by a man who accused him of being a narcotics officer. The man then punched Black in the face, police said.

Black, 19, denied the man's charges and walked away from the fight, his cover intact.

"I was able to make my first 'buy' five days after that assault," said Black, who became the talk of Howard County after 11 high school students and one adult were arrested last month on the evidence he collected. Few Howard residents know Black by name. Even fewer will see his face; for his safety, police would not allow Black to be photographed during a recent interview.

His work, however, has won the praise of his superiors, who say such undercover work is the best attack on the widespread use of drugs by students in this largely rural county. Educators and parents say they would like to see more operations like Black's, the third such undercover investigation in five years.

"The arrested students may perceive Tim as a snitch, a rat, a 'narc,'" said Detective Charles S. Gable, who supervised Black's investigation. "But the parents, the teachers and the Board of Education want us in the schools."

With his long hair, sloppy clothes and bottomless cash reserve with which to buy drugs, Black appeared to be just another 11th grader at the two schools he attended--Hammond in south central Howard and the county vocational-technical center in Columbia. Police had targeted six suspected drug dealers, and Black got to know them and their friends. Four of the suspected dealers were later arrested, police said.

In all, Black made 17 purchases of small amounts of drugs, once buying hashish from a 14-year-old freshman, he and police said.

He said he witnessed student drug sales during classes and on school buses and smoked marijuana to preserve his fake identity. Gable said Black underwent a two-week course on drugs, their street nicknames, plus tips on smoking marijuana without inhaling it. Black does not even smoke tobacco, Gable noted.

"In every case, I was the one who was approached about a sale," said Black, a wiry Baltimore-area native who now sports a short, blow-dry haircut and the mustache he was ordered to shave off to look younger.

As a member of the county cadet program, which gives high school graduates aged 18 to 21 experience in routine police work, Black carries no firearm and has no arrest power. Evidence collected by Black, who earns $13,100 annually, enabled police to seek warrants for arrests in the case.

Police had alerted the Hammond and vocational school principals to Black's assignment and provided him with a cover story: He had been kicked out of his parents' house near Baltimore and was living with a relative near Ellicott City.

Black said he bought marijuana, LSD and tablets containing Diazepam, the main ingredient in Valium, from students in five Howard high schools, including the two he attended as well as Atholton, Centennial and Howard high schools.

The 10 students who are juveniles have been released in the custody of their parents, while an 18-year-old Hammond senior who was involved with an LSD sale in Prince George's County is free on his own recognizance, police said.

A second adult, a 20-year-old former Howard High student, also was arrested on drug possession and distribution charges, police said. The man who struck Black is still at large; police say they expect to make others arrests in the case. "Compared to some urban school systems, our drug problems are very small," education department spokesman Paul F. Rhetts said, "but we would rather have police in our schools than drugs."

Black said he had no qualms about investigating students only two years younger than himself. "I figured this kind of work had to start sometime," said Black, who plans a career in law enforcement. "I will be arresting people my age and older someday. It's just part of the job."

In the last week of his assignment, vandals who by then were suspicious of Black smashed a door and taillight of his car. Repairs cost nearly $600; the $200 spent in drug purchases and other expenses brought the cost of the police operation to roughly $1,000, police said.

Howard County police will probably mount future undercover investigations in the schools as part of ongoing probes into major drug suppliers in the area, Gable indicated. "Our attempt is to move up the line . . . to the major distributors," said Gable, a nine-year veteran of the force.

Tim Black, however, has for the moment outlived his usefulness as an undercover agent, which is fine by him, he said. "After it was all over," Black said, "it read on paper better than it really was. A lot of it was long, boring days--sitting in class and all that."

This week, Black was scheduled to start working traffic details around Howard County.