Poor children who go to affluent state schools are "derided, abused and shunned" by snobbish, richer pupils, a top government official has warned, and their teachers fail to meet their needs.
Those from deprived backgrounds are "swamped" by their wealthier peers and suffer bullying, according to Sue Hackman, chief adviser on school standards for the Department for Children, Schools and Families.
Writing in a DCSF report, Pockets of Poverty, Ms Hackman says the pupils also feel socially "ill at ease, or embarrassed or out of their element" in these schools.
She says that a recent investigation by the department showed that, in addition to being snubbed by their peers, when poorer pupils are a minority teachers do not see them as a priority. "Doubly disadvantaged", these children have among the highest rates of underachievement in England.
Ms Hackman says this situation is reversed in poorer schools which have a large proportion of children who qualify for free school meals (FSMs), where their progress is "often a significant factor in the schools' overall success".
Disadvantaged pupils carry with them awareness of being "less affluent, less mobile, less experienced and less 'savvy' than others who eat better, dress better and are in every sense better off than they are", she said.
She added: "They can find themselves derided or shunned by other pupils who recognise them as different and who have invented a range of abusive terms for poor pupils.
"These pupils are actually doubly disadvantaged. They experience all the difficulties associated with their comparative poverty and they find themselves in a significant minority, having to live in the midst of a community and school population who are more affluent, perhaps considerably more affluent, than they are."
Teachers have been sent new advice on the issue, which Ms Hackman says is a "significant cause for concern" for ministers and civil servants. Other "achievement gaps" have closed, but FSM pupils are still only half as likely to get five good GCSE grades as those who do not qualify for the benefit.
Around a third of the poorest children attend schools where the majority of pupils are from wealthy homes.
Ms Hackman said teachers should create a school community which values working-class pupils. The guidance says affluent schools should identify poorer pupils, track their progress carefully and act if they are not meeting targets.
"Being aware of their situation is a starting point, but we need to do more than that," she said. "We have encountered schools which are reluctant to identify FSM pupils for fear we may return the old shames of dinner tickets, separate queues and a new pauperism. This will not happen; denial is the greater risk."