The study, published in the American Journal of Sociology, found that peer aggression occurred at higher rates between friends, and friends-of-friends, than between those not closely tied.

"People often assume that bullying occurs between relative strangers, or that it targets those on the fringes of the social network," said researcher Diane Felmlee from the Pennsylvania State University in the US.

"Those do occur, but in our study, we find that the rate of peer aggression is significantly higher between those students who are closely linked," Felmlee added.

According to the National Center for Education Statistics, about 20 per cent of students aged 12 to 18 report being bullied at school during the school year. And while many anti-bullying programmes exist, the researchers said they are not always effective.

For the study, the team used data from more than 3,000 students that was collected in waves, starting when the students were in grades six, seven and eight and finishing when they were in grades eight, nine and 10, respectively.

The researchers constructed "aggression networks" by asking students to nominate up to five classmates who had picked on or been mean to them, allowing the researchers to identify both bullies and victims.

One of the students who reported being the victim of a friend noted, "Sometimes your own friends bully you. I don't understand why, why my friends do this to me."

Additionally, participants who were friends in the fall of the school year were over three times as likely to bully or victimize the other by the spring of the same school year.

Being bullied by a friend was also linked to significant increases in anxiety and depression and lower levels of school attachment, the researchers said.