Students of the Saraswati Shishu Mandir public school complained of uneasiness, vomiting, nausea, itching and tears in the eyes.
Before the school management could react to the situation, more than two dozen students had fainted, an official informed IANS.
They were rushed to the district hospital. A dozen students in serious condition were referred to Muzaffarnagar and Meerut hospitals.
Senior district officials including the District Magistrate and the Superintendent of Police (SP) rushed to the school as news of the incident spread.
Panic-stricken parents rushed to the school and took their wards away. Some were taken to private hospitals for treatment.
Total chaos prevailed in the school for over two hours as medical teams rushed to the school late and the children were left to fend for themselves, some parents said.
There is no medical emergency facility at the school, they added.
Local residents and parents also accused the district administration of turning a blind eye to their demands in the past against the sugar mills which frequently release chemical gas that pose serious health hazard to the children and others in the vicinity.
The owner of the sugar mill was absconding, the police said.