Nearly a quarter of London primary schools are in areas where air pollution limits were broken in 2010, according to a report that the capital's new mayor claims was not published by his predecessor Boris Johnson.
The report, completed in 2013, also shows that 82% of the schools affected by excess nitrogen dioxide were in deprived areas.
But London mayor Sadiq Khan's office said the former mayoralty "decided to cover it up", and pledged to tackle the capital's pollution problems.
Emissions experts used data from the Environment Research Group at Kings College London and found that 433 of the capital's 1,777 primary schools (24%) were in locations where average concentrations exceed the nitrogen dioxide EU limit.
Authors Katie King and Sean Healy noted the "clear disparity" between "deprived schools" - where more than 40% of pupils are eligible for free meals - and those not classed as "deprived".
They wrote: "In combination with other measures to reduce pollution, we recommend that consideration should be given as to what further action can be taken at schools, particularly those in deprived areas."
The report also found that people living in the most deprived areas were on average more exposed to poor air quality than those in less deprived areas.
The Greater London Authority commissioned the report and it was completed by Oxford-based environmental consultancy Aether.
In January, a spokeswoman for Mr Johnson said that he was "leading the most ambitious and comprehensive package of measures in the world to improve London's air quality".
A spokesman for Mr Khan said: "This shocking report reveals a snapshot of the true impact that our polluted air has on some of London's most vulnerable communities.
"It is difficult to understand why the last mayoralty decided to cover it up and not fully release it in 2013 - they clearly didn't want Londoners to know the dire state of pollution in the capital.
"The mayor is fully committed to cleaning up our air and protecting Londoners' health, and is shocked to learn that important scientific evidence like this has been locked and ignored at City Hall."