The students of Saraswati Shishu Mandir were taken to a government hospital after they suffering breathlessness, nausea, stomach ache, giddiness and a burning in their eyes.
Over two dozen children are believed to be seriously ill and some of them have been referred to a bigger hospital in Meerut.
"Between 30 and 35 children are in a serious condition, 15 have been referred to Meerut," said Surjeet Singh, a senior district official.
Doctors say none of the other children are seriously affected. But at the hospital, there was panic among parents and teachers.
In visuals, little children were seen sitting in groups at the hospital after first aid.
The childrens' parents say waste from a sugar factory is dumped at a site not far from the school.
When the children were heading to school today, factory workers were in the process of burning the waste products, which allegedly gave off toxic fumes.
While a few children reportedly fell unconscious, others were taken ill when they reached school.
When hundreds of children appeared to collapse with the symptoms of the toxic gas, teachers took them to the hospital.
Frantic parents soon arrived and started protesting about the danger to their children's lives from the factory.