Daily marijuana use by teen-agers -- and peer acceptance of daily pot smoking -- is decreasing, according to the University of Michigan's annual survey of high-school seniors.

"Peer norms are shifting away from acceptance," says Lloyd Johnston of Michigan's Institute for Social Research, which has surveyed a nationally-representative sample of 130 high schools (17,000 seniors) each year since 1975.

"The proportions of high-school seniors who say they smoke marijuana on a daily or near-daily basis has begun to decline. And we have observed a gradual increase in the proportion of seniors who themselves disapprove of regular marijuana use (75 percent disapproved in 1980 vs. 66 percent in 1977).

Concern about health appears to be responsible for the decline. On a list of reasons why students decided not to use marijuana, more students mentioned possible physical and psychological effects than cited moral, legal or social constraints. Also ranked high -- "concern about loss of energy."

"There also appears to be decreased acceptance of occasional and even experimental marijuana use," says Johnston, "although these lesser degrees of involvement with the drug are far more widely accepted than regular use."

While the general situation with marijuana seems to be improving, he says, "I do not want to leave the impression that the problem is near to disappearing."

In the class of 1980:

* 60 percent of seniors reported some experience with marijuana.

* Nearly half of all seniors had smoked it in the last year.

* 9 percent were smoking it daily.