Alexandria police arrested two T.C. Williams High School students on drug charges yesterday, ending a two-month undercover investigation aimed at assessing the amount of drug use at the school.

One of the students, a 17-year-old senior, was charged with two counts of selling PCP and one count of selling marijuana. The other, a 17-year-old sophomore, was charged with one count of selling PCP.

"We are grateful to the Alexandria Police Department for the way they conducted this investigation," said T.C. Williams Principal John Porter. "We are also encouraged by the number of students who told the undercover police officers that they didn't want to have anything to do with drugs."

The undercover operation, in which two police officers had posed as students since April 1, was started after parents, School Board members and community groups expressed concern about the frequency of drug use among the 2,400 students at T.C. Williams.

"There is not nearly as much drug activity in the school as we had anticipated," said Lt. Arthur Bratcher, head of the police vice-narcotics unit. "The officers there got a good indication that a lot of kids were turned off by drugs."

He added that it was the first time the police had engaged in any undercover operation in the Alexandria school system.

Both of the students charged yesterday were released into the custody of their parents pending a hearing in Alexandria Juvenile and Domestic Relations Court. No date has been set for the hearing.