When most television sitcoms and teen movies portray social outcasts, they usually show them excelling in school despite the public peer rejection they endure in the hallways and lunchrooms of America’s high schools. In reality, those teens are not excelling, and peer rejection is no laughing matter.

The Washington Post reports that researchers who followed 380 Midwestern children from the ages of 5 to 11 found that those who were chronically rejected by their classmates were more likely to withdraw from school activities and scored lower on standardized tests than their more popular peers.

“We’re talking about kids whose classmates don’t let them sit with them in the cafeteria,” said lead researcher Eric S. Buhs, an assistant professor of educational psychology at the University of Nebraska-Lincoln. “This is what happens when a whole group demonstrates, ‘We don’t want you around.’ ”

Peer-group rejection, Buhs and his co-authors report in a study funded by a grant from the National Institutes of Health, starts as early as kindergarten. It appears to affect boys and girls equally. And it often triggers a vicious circle that can cause long-term psychological damage and impair a child’s academic performance.

Exclusion obviously makes it difficult for a child to join group activities, so the victim disengages from school as a way of avoiding further abuse. Withdrawal acts as a “persistent signal to classmates” that rejected children are not members of the group and reinforces the ostracism, noted the researchers, whose study appears in the current issue of the Journal of Educational Psychology. Academic achievement can be hampered by diminished participation in class.

Buhs’ team found that students who were rejected by their peers in kindergarten tended to become children who were chronically rejected in older grades. By fourth grade they scored measurably lower on standardized reading and math tests than their classmates.

“Social isolation is one of the most devastating things you can do to a human being; I don’t care how old you are,” said Rosalind Wiseman, a veteran educator in Washington and the author of “Queen Bees and Wannabes,” the bestselling book about girls and cliques that became the basis for the movie “Mean Girls.”