Ofsted ratings may be unfair, with secondary schools in the poorest areas much more likely to be declared failing than those serving wealthy communities, a report suggests.
Inspectors can be too generous when judging schools with low numbers of poor pupils and too harsh on those that have a large proportion of youngsters on free school meals (FSM), it is claimed.
However, Ofsted said that inspectors looked beyond raw attainment when making judgments and added that studies have shown that schools in deprived areas face tougher challenges, such as teacher recruitment.
The report, by the Education Policy Institute, looked at the inspection results of schools across England.
It found that secondary schools where up to 5% of pupils were eligible for free dinners - a key measure of poverty - were more than three times as likely (48% against 14%) to be judged outstanding by Ofsted compared with those with at least 23% of poor students.
In addition, about 15% of secondaries with the highest numbers of poor pupils were rated as inadequate, compared with only 1% of those schools with the fewest disadvantaged youngsters.
Among primary schools, those with large numbers of FSM students were half as likely (11% against 25%) as those with low numbers of poor children to be judged outstanding.
Researchers also examined schools' value-added data, which recognises the work schools do to help pupils make progress and achieve, alongside Ofsted ratings.
It concludes there are inconsistencies between identified drops in academic standards and Ofsted grades.
For example, the study found that 1,221 primaries and 228 secondaries were identified in the EPI's analysis as having deteriorated substantially based on their value-added data between their previous and latest inspections.
But of these, 962 primaries and 152 secondaries received the same, or a higher, rating in their latest inspection.
Separately, the EPI said it calculated that, based on value-added progress, about 500 too many mainstream state schools with the lowest proportions of poor pupils had been rated outstanding.
"Based on value-added alone, and all else being equal, our analysis suggests that there are around 360 too few 'outstanding' judgements among schools with the highest proportions of FSM," the EPI said.
"There may be aspects of school effectiveness other than academic performance that affect these judgments in some cases."
Report author Jo Hutchinson, said: "Our research suggests that the Ofsted inspection system may not be fully fair and equitable to schools with challenging intakes.
"Schools with the fewest disadvantaged pupils are significantly more likely to be judged 'good' or 'outstanding', and a significant proportion of 'good' and 'outstanding' schools are not downgraded, despite a substantial deterioration in their academic performance."
"While there are potential explanations for lower overall rating of some schools with challenging intakes, including higher rates of teacher turnover and fewer experienced teachers, when we benchmark the distribution of Ofsted judgments against the value-added progress of pupils, some of the outcomes we observe are not explained by levels academic performance in the lead-up to the inspection.
"In particular, our analysis suggests there may be too many 'outstanding' judgments for schools with very low levels of deprivation and for schools with very few pupils with low prior attainment.
"While factors other than academic performance can potentially explain lower judgments for schools with challenging intakes, it is not clear why schools that serve the fewest disadvantaged and low-attaining children should be judged 'outstanding' despite value-added progress levels that do not indicate this to be the case."
An Ofsted spokeswoman said: "Inspectors do look beyond raw attainment when making their judgments.
"Indeed, under Sir Michael Wilshaw we have increasingly focused on the progress children make from their different starting points.
"As a result, inspectors do mark down coasting schools in leafy suburbs where we see pupils not making as much progress as they should. Similarly, we do recognise schools in more deprived areas where children are making good progress.
"Studies have shown that many schools in deprived communities face tougher challenges including, for example, teacher recruitment.
"As Sir Michael has said, the challenge is to get sufficient good teachers to work in the areas where they are needed most.
"As this report acknowledges, inspection judgments are based on a wealth of evidence not just data, and certainly not just one year's worth.
"Inspectors use their professional judgment to look at performance over time, the progress being made by pupils currently in a school and the effectiveness of leadership and management.
"That means we would not automatically mark down a school for a 'sudden decline' in a single performance measure in a single year, as this report seems to suggest we should, if other evidence shows a school remains good or outstanding overall."
An Ofsted spokeswoman added: "It is also important to note that our inspection methodology, including the frequency of inspection, has changed significantly over the 10 year period covered by this report and that the report does not reflect current inspection practice as inspection data from the last school year is not included.
"As the report notes, in 2012, the government removed 'outstanding' schools from routine inspection altogether, unless their performance deteriorated significantly or there were other causes of concern, such as parental complaints or safeguarding issues."
Malcolm Trobe, interim general secretary of the Association of School and College Leaders, said: "These findings suggest that schools in disadvantaged areas, which already face significant challenges, may be unfairly treated by the inspection system. This is a matter of great concern, and we recommend that the incoming Ofsted chief inspector Amanda Spielman investigates the situation."