Thousands of England's schools are segregated by ethnic or social status, according to research.
It reveals that in 2016, more than a quarter of primaries (26%) and around two fifths (40.6%) of secondaries were ethnically segregated.
And nearly three in 10 (29.6%) of primary schools and over a quarter of secondary schools (27.6%) are split by social background.
This means that many schools across the country have pupil populations made up overwhelmingly of white British, or ethnic minority youngsters, or have large numbers of youngsters from either rich or poor homes.
The findings show that more needs to be done to make school intakes more representative of their local communities, according to The Challenge charity, which carried out the research.
Researchers from the charity, working with the iCoCo Foundation and SchoolDash, examined how segregated a school was by comparing its numbers of free school meals and white British pupils with those of the 10 schools closest to them.
They used official data for the years 2011 to 2016, covering more than 20,000 state schools.
A school was considered "segregated" if the proportion of ethnic minority pupils or pupils on free school meals was very different to the proportions at the 10 closest schools.
The study found that secondary schools are more likely to be segregated by ethnicity while primaries are more likely to be divided along socio-economic lines.
Primary faith schools are more ethnically segregated than those of no faith (29% against 25%) when compared with neighbouring schools, the study found.
Primary schools with a religious character were also more likely to have a wealthier student population, with over one in four (27%) having significantly fewer disadvantaged pupils than other nearby schools, compared with 17% of non-faith primaries.
At secondary level, schools rated as "inadequate" tended to be more ethnically segregated, while those judged "outstanding" by inspectors were more likely to have a representative mix of pupils, compared with neighbouring schools.
And grammars - a school type earmarked by the Government for expansion - were severely segregated by social background.
Some 98% of these selective schools had low numbers of poorer pupils, compared with their local schools, and none had pupil populations with high numbers of free school meal students.
The study also suggests that in some areas the situation is worsening - with primaries becoming more ethnically segregated in the last five years in over half of the 150 areas analysed.
Jon Yates, director of The Challenge, said: "This study shows far more needs to be done to make sure school intakes are representative of local communities.
"As the Government's Casey Review pointed out, segregation is at a 'worrying level' in parts of the country."
Local and national government needs to commit to doing more to reduce school segregation, he said.
"We know that when communities live separately, anxiety and prejudice flourish, whereas when people from different backgrounds mix, it leads to more trusting and cohesive communities and opens up opportunities for social mobility.
"We urge local authorities, faith schools and academy chains to consider the impact admissions policies have upon neighbouring schools and put policies in place that encourage better school and community integration."
A Department for Education spokesman said: "We expect all schools to promote social integration and the fundamental British values of democracy, the rule of law, individual liberty, and mutual respect for different faiths and beliefs.
"Our free schools programme already encourages applications for free schools which aim to bring together pupils from different ethnic or faith groups, and our consultation, Schools That Work For Everyone, includes faith schools setting up twinning arrangements with others not of their religion so that pupils mix with children from different communities and backgrounds.
"But we know there is more to do. The Casey Review highlighted a number of issues around levels of ethnic segregation in school intakes in some areas of the country. The Government is considering the review and its recommendations and will respond in due course."