Use of illegal drugs among high school seniors last year dropped to the lowest level since the annual survey of students began in 1975. Fewer than half reported ever trying illicit drugs and the number who said they smoked crack cocaine dropped sharply, according to the survey of 17,000 seniors released yesterday.

The findings fit a pattern that began to emerge in the late 1980s as public concern about the hazards of drug use began to take hold. Federal officials, who financed the survey, said both public education programs and tougher law enforcement measures contributed to the latest decline.

In the spring of 1990, about 48 percent of high school seniors said they had used an illegal drug at least once, down from a high of almost 66 percent in 1982. It was the first time since the annual surveys began in 1975 that a majority of students said they had never used an illegal drug.

But the 1990 results for the legal drugs of tobacco and alcohol were not as encouraging. Those remained the drugs most often used by high school seniors.

And others were skeptical of the findings. Rep. Charles B. Rangel (D-N.Y.), chairman of the House Select Committee on Narcotics Abuse and Control, said the survey may create a misleading impression because it did not cover high school dropouts. About 29 percent of students do not complete high school.

Rangel said health professionals, criminal justice officials and drug treatment workers "do not report seeing a lessening of the drug crisis in the country."

The survey showed either decreases or no changes in student use of every category of drugs, with the biggest drop reported in crack use. The proportion who reported ingesting crack in the last year fell to 1.9 percent, compared to 3.1 percent in 1989. Crack use within the last month was admitted by .7 of a percent, a 50 percent decrease from the 1.4 percent in 1989.

Those decreases were the largest found since the survey first inquired about crack in 1986, not long after use of the concentrated form of cocaine became known.

Similar declines were reported in the use of any kind of cocaine, whose popularity among high school students the survey has found to be diminishing since 1986. Cocaine use in the last year dropped from 6.5 percent to 5.3 percent, while use in the last month fell from 2.8 percent to 1.9 percent.

Lloyd Johnston, the principal investigator and a social scientist at the University of Michigan, described the decreases in cocaine and crack use as "quite sharp." Frederick Goodwin, director of the Alcohol, Drug Abuse and Mental Health Administration said the figures on cocaine use represented "accelerating trends" of decreasing use.

Health and Human Services Secretary Louis W. Sullivan said the survey results were a "significant milestone" of success "convincing students that drugs are not a rite of passage, but a road to disaster."

"Overall, these numbers document millions of individual decisions to reject the peer pressure and false claims made by the drug culture," Sullivan said.

Goodwin said the survey said declining drug use was consistent regardless of family income or residency in urban or suburban areas. As for racial factors, he said the data showed "if anything, blacks are less likely than whites to be using drugs, a trend that's been consistent over many years."

Sullivan, who has crusaded against cigarette smoking, said he was disturbed that prevalence of that habit among students has been virtually unchanged for seven years.

There were decreases reported in the consumption of alcoholic beverages in the past year or month, but the percentages of problem drinkers -- those who take alcohol daily or in binges -- showed no significant change.