A survey of primary teachers reveals that a third have noticed an increase in loneliness among pupils since they began teaching. The same number say that children find it more difficult to relate to others and to make friends.
The survey of 150 teachers was carried out by the charity Save the Children.
Teachers were also concerned that bullying was on the rise. A quarter said that verbal abuse towards other pupils had increased since they began teaching. Almost as many said that physical violence had increased.
Lorna Redden of Save the Children said: "Friendship should not be taken for granted. If children feel left out and rejected, that bruise can take a while to mend."
Jo Holden, a Year 6 teacher at Bond primary in Mitcham, Surrey, uses drama to teach pupils about the importance of friendship. The school also emphasises the value of quietness in corridors and in the dining room.
"We're teaching them to deal with their emotions," she said. "But if the school in general is a nicer place, you don't get things like bullying.
It's all interlinked."
Ten-year-old Lisa Vella agrees. "It's important to be friendly, because it makes people sad if you're not," she said.
"Once, people called me names and said I had nits, and it was very horrible. If you don't like someone, just leave them alone."
The survey was carried out to mark Friendship Friday, an annual fundraising day for Save the Children, to be held on May 25.