Greenpeace collected air samples from schools across the capital between October to November last year.
On the days when PM 2.5 levels (particulate matter with a diameter below 2.5 microns, responsible for pollution), were above the prescribed levels, there were "alarming" levels of heavy metals in indoor air, the study said.
PM 2.5 concentrations from monitors installed inside classrooms showed indoor air quality five times above the Indian safety limits and as much as 11 times above the World Health Organization's safety limits.
The average PM 2.5 levels in these schools stood at 293 units, against the Indian normal of 60 units.
The same samples, when tested for elemental composition of PM 2.5, showed dangerous levels of heavy metals such as arsenic, cadmium, lead and nickel which are carcinogenic and detrimental to health.
The detected heavy metals such as lead and manganese are neurotoxic and affect the cognitive and motoric development of children in particular. Arsenic, cadmium, nickel and chromium cause cancer in humans.
"The result signifies that school children are exposed to exceeding levels of heavy metals that increase the risk of cancer and developmental problems. Higher the PM 2.5 concentrations, higher will be the exposure to heavy metals," Sunil Dahiya of Greenpeace India said in a statement.
About the source of the heavy metal presence in the air, Dahiya said: "most of these particles are attached to the anthropogenic impacts caused due to fossil fuel (coal and oil) burnt for energy and transportation sector".
Delhi needs to take stringent measures to reduce the air pollution exposure to children. Urgent action such as shutting down schools and restricting outdoor activities should take place, experts say. This practice must be expanded to include advice on precautionary measures to other sensitive groups by the government.
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