Me level 4, you level 2 = end of friendship
Primary pupils see failure to meet test targets as a sign that they are bad people and are reluctant to be friends with those who do not perform as well as themselves.
Academics from London university's Institute of Education discovered children identify so strongly with the level they are likely to achieve in tests for 11-year-olds that they feel tarnished by failure if they under-perform.
Tamara Bibby, who led the research team, said: "Children start to think of themselves as levels. And it's wrapped up with morality and goodness. Good people work hard and listen in class.
"If it suddenly becomes clear your mate gets lower levels than you, are they a good person? It can put real pressure on a friendship."
Friendships often suffered, with higher-achieving children no longer willing to spend time with their under-performing, and therefore bad, friends.
Dr Bibby quotes a conversation in which a 10-year-old boy struggled to justify his friendship with a lower-achieving classmate.
The boy said:: "He's got level 3. That means he's working hard, but not hard enough. But he is a good boy, so he will do better."
Dr Bibby observed a class of key stage 2 pupils from the end of Year 4 through to Christmas in Year 6. She found that children also had very definite ideas about what made a good student - "A good student is one who's quiet and compliant and thoughtful. And therefore probably a girl.
The perception is that girls are good and boys are naughty."
For boys, therefore, becoming good would involve losing a part of themselves. One boy said: "All the girls' handwriting is beautiful. Girls do that at home. We go home and play football."
Pupils also suffered from a lack of control over their own time. While the pressures of the curriculum led teachers to see time as a commodity that must not be wasted by pupils, the children felt their own time could be wasted.
"Pupils are told to hurry up, then wait," Dr Bibby said. "That leads to resentment."
While adults in an office might be able to cope with an unpleasant encounter with a co-worker by taking a break to moan to a colleague or make a cup of tea, no such options were available to children.
The school system often frustrated pupils who wanted to spend time developing and understanding friendships. "For children, that's exactly what school is all about," Dr Bibby said.