Pupils are being taught in classrooms where the air is so polluted it breaches World Health Organisation (WHO) guidelines, a study has found.
The work found that in some cases pollution levels inside the schools were higher than outside.
The study, which looked at five primary schools and a nursery in London, was published as the capital's mayor Sadiq Khan launched a £1 million fund to protect pupils from toxic air.
The work by researchers from University College London and the University of Cambridge was commissioned by Mr Khan.
It highlighted the levels of tiny particulate matter in the air inside the schools - at PM2.5 and the larger PM10 scale.
When classrooms were occupied, indoor PM10 levels were "consistently higher than outdoors".
The report found that mean PM10 and PM2.5 levels recorded in all classrooms in both heated and unheated seasons were higher than 20 mg/m3 and 10 mg/m3 respectively, "indicating that annual personal exposure to PM in the classroom may be higher than WHO 2010 guidelines".
"In most classrooms, PM concentrations were above daily guideline values," the report said.
School-aged children are more vulnerable to airborne pollutants than adults not only because of their narrower airways, but also because they generally breathe more air per kilogram of body weight.
"The exposure of children's developing lungs to air pollution can result in reduced lung function that persists through to adulthood, increasing susceptibility to respiratory and cardiovascular diseases," the report said.
The researchers also highlighted that the UK has the "highest prevalence of childhood asthma" in Europe.
A review of existing studies concluded that "children living or attending schools near high traffic density roads were exposed to higher levels of motor vehicle exhaust gases, and had higher incidence and prevalence of childhood asthma and wheeze".
The study found differences in pollution levels between classrooms depended on a range of factors, including building characteristics, design and maintenance.
A significant proportion of indoor air pollution was due to outdoor air pollution.
For nitrogen dioxide (NO2), which was strongly related to the risk of asthma attacks and asthmatic symptoms, outdoor sources accounted for 84% of the variation between classrooms, highlighting the importance of tackling emissions from road traffic.
In another element of Mr Khan's work to tackle London's dirty air, 50 schools across the capital were audited to assess their air quality and recommendations were made to improve the situation.
The £1 million fund will help the worst affected schools bring in changes immediately while 20 nurseries will also be provided with air quality audits and indoor air filters.
Mr Khan said: "Air pollution is a national health crisis that is putting the health of children at risk.
"As mayor, I've moved fast in London to implement the most ambitious plans to tackle air pollution of any major city in the world.
"This includes cleaning up our bus and taxi fleets, bringing forward the introduction of the world's first ultra-low emission zone and introducing the toxicity charge - T-Charge - for the oldest polluting vehicles in central London.
"But I can't do this alone. The Government must step up and act with more urgency if we are going to tackle London's filthy air once and for all."