An Alexandria student became the first target in Northern Virginia last week of an 18-month-old state law forbidding anyone to carry an unauthorized beeper on a public school grounds, according to school and law enforcement officials.

The 15-year-old was arrested on the misdemeanor charge Thursday after Hector Montenegro, principal of Francis C. Hammond Junior High School, informed police that the student had refused to heed warnings to stop carrying a beeper.

The state law was one of several measures, including one outlawing drug sales within 1,000 feet of a school, passed during the 1989 legislative session and aimed at slowing the drug trade around public schools. Drug-related trials in Alexandria and elsewhere have shown that beepers are used frequently to arrange drug sales.

The student, who was arraigned and released to his parents' custody, did not have any drugs, police said.

Although officials from several area school districts reported yesterday that beepers have been found in schools, none described beepers as a significant problem.

Alexandria Commonwealth's Attorney John E. Kloch said yesterday that though the law has not been enforced frequently, it gives school administrators a tool for keeping beepers off campus before they become a problem. "It's largely a preventive piece of legislation," said Kloch. "I don't expect a lot of prosecutions."

Kloch said that the law, punishable by a maximum of one year in jail and a fine of $2,500, allows administrators to confiscate beepers or to ask for police action when violations are flagrant.

Montenegro said yesterday that Hammond has had about 10 incidents in two years in which students were warned to keep beepers off campus. Two cases involved the 15-year-old arrested last week.

Montenegro also said that none of the students found with beepers had drugs or weapons in their possession. He said Hammond will continue to enforce the ban, though he added that "in many of these cases students bring these things to school strictly for show."

David R. Rorick, spokesman for the Arlington schools, said that Arlington has not experienced a problem with beepers and regulates their use through a general policy prohibiting students from creating a disruption in class.

Rorick said some "yuppie students" might carry beepers to keep in touch with friends or parents. He said many reports that students carried beepers turned out to be false. "You look into it and it doesn't seem to be there," Rorick said.

District schools have had a policy banning beepers and other electronic communications devices on campuses since 1986. But Pat Lambe, spokeswoman for the schools, said she had never "gotten a call on the issue of the possession of beepers."