Marijuana use by teen-agers and preteens has been increasing sharply, a trend called "alarming" yesterday by Health, Education and Welfare Secretary Joseph A. Califano Jr.

"It is sheer folly," he said, "for millions of young Americans to indulge in a drug while so little is known about its long-term consequences, and when much of what is known" suggests possible lung problems and "psychomotor impairment"-ill effects on the brain and nerves-if the drug is used long enough.

Between 1976 and 1977, according to latest federal surveys, the number of youths aged 12 to 17 who were current marijuana users increased by nearly a third, from 12.4 to 16.1 percent. [Current use was described as having smoked pot within the previous 30 days.] Between 1975 and 1977, the number of high school seniors who used marijuana daily increased from 6 to 9 percent.

The figures appear in the HEW secretary's annual report to Congress on marijuana and health. "Unfortunately," the report emphasizes, it may take many years for research scientists to say positively whether marijuana is a serious public health threat.

However, said Califano: "Because of the rapid rate of physical and psychological development in young people, the report suggests that the young may be especially vulnerable . . . The report provides additional evidence that marijuana intoxication impairs driving, by reducing perceptual acuity and dulling reflexes . . . This [too] is especially serious for the young since accidents are already the leading cause of adolescent death and injury."

Looking more closely at marijuana use among the young, the report finds:

Young adulthood, between ages 18 and 25, continues to be the period of peak use. More than one in four 18-to-25-years-olds had used the drug in the month preceding the most recent survey.

Among children 12 and 13, 8 percent had some experience with marijuana, and 4 percent reported current use. At ages 14 and 15, 29 percent reported some use, and 15 percent current use.

Some experience was reported by 47 percent of the 16- and 17-year-olds, 59 percent of 18-to-21-year-olds and 62 percent of those 22 to 25. Current use was reported by 29 percent of those 16 and 17, 31 percent of those 18 to 21 and 24 percent of those 22 to 25.

Roughly two adolescent girls used the drug for every three boys, a far higher percentage of females than the one woman for every two men 18 and older.

Among adolescents, use by whites slightly exceeded use by other races. For those over 18, use was about equal.

Many teachers and most psychologists have said some adolescents are knocking themselves out with marijuana-and sometimes with marijuana and alcohol-so mucht that they become zombies in school and cannot learn. The HEW report said it is still uncertain whether apathy, listlessness and other personality problems are a cause or the result of marijuana use.

But the report called the combination of alcohol and marijuana particularly dangerous. It also warned of possible ill effects on babies whose mothers use the drug during pregnancy.