Frequent television viewing is increasingly being linked to poorer achievement in school. ''There is at this point massive research evidence that kids who spend more time watching TV will get lower test scores and lower grades,'' says Michael Morgan, a researcher on the effects of television, who is at the University of Pennsylvania's Annenberg School of Communications. He was one of the principal authors of a 10-year summary of research on television and behavior, published last year by the National Institute of Mental Health.
Reading skills are the main achievement affected, he said. A study just being concluded at Yale University finds children aged 7 to 9 who are heavy viewers got 10 to 20 percent lower reading scores than children who watched less.
But not every child is affected the same way, Mr. Morgan says. And the link between viewing and poorer grades is not so clear for children below the fifth grade, he notes.
No one is quite sure just why the relationship can appear so strong. There is agreement, however, that TV viewing is only one of the elements involved for any individual child. Home life, friends, and other factors are also important, researchers say.
But without waiting for the definitive answers, researchers, as well as spokesmen for the television industry, are calling for parents and teachers to help in devising ways to take advantage of the positive educational potential of television.
Leaving children alone to watch as much as they want and what they want is not good enough, according to specialists on the effects of television on children's learning.
In some instances - for example, with younger children, children learning English, or children in a culturally deprived setting - television may actually help learning. Even here, however, the benefits tend to disappear at an older age.
Television is ''a major potential for learning,'' says Thomas Cook, professor of psychology and public policy at Northwestern University in Evanston, Ill., a noted researcher on the effect of television on children. ''They (children) spend more time watching TV than in school. The national average of all ages of children is about 25 hours a week of TV viewing, year round.''
But many things influence a child's learning, he says, including home background, parental actions, friends, and teachers. Also important are the reasons a child watches television, he says.
Isolating the television-learning link from these other effects is ''an impossible order'' at present, he said in a telephone interview.
''It (television viewing) may increase achievement of younger kids but decrease the achievement of older kids, Professor Cook says.
Steven Miller of Television Information Office, a public relations firm for the industry, says: ''We don't feel TV should be blamed for the decline in scores. He says television is one factor in learning, but not the only factor. People like to use television as a scapegoat.''
Mr. Miller's firm stresses greater parent and teacher involvement to take advantage of the positive educational potential of TV.
From his own studies and from his review of others' work, Dr. Morgan finds heavy television viewing and poorer grades are most closely linked among:
* Children with high IQs. (Children with low IQs who were heavy viewers actually scored better on achievement tests than light viewers with the same IQs.)
* Less popular students.
* Children whose parents did less to select and interpret programs and limit viewing time.
* Children with higher educational and occupational aspirations. Heavy TV viewing seems to lower these aspirations to the ''mainstream,'' he says.
* Children who watched television more attentively. Those who watched while eating, talking, or intermittently reading were less likely to have poorer grades.
''The lighter the television viewing, the less effect it has,'' says Eli Rubinstein, an adjunct research professor at the University of North Carolina. ''The kids who tend to be light viewers are probably into other things.''
Carefully designed programming can constructively spur a child's imagination and social behavior, but there is ''practically none'' of that on the networks, says Jerome Singer, co-director of the Family Television Research and Consultation Center at Yale.
He urges parents to take ''seriously'' the issue of what their children watch on television and how much.