The students of Sarvodaya Gyan Public School in Deoband met the accident this morning, Additional Police Superintendent (Rural) Jagdish Sharma said.
There were 20 children in the vehicle. Suddenly the van's radiator burst and boiling water got poured over half a dozen students injuring them, he said.
Three out of the six students- Vasi, Ujmaa and Kushagra - are critically injured and are taken to a burn specialist doctor in Gokulpur in Dehradun, they said.
Some parents also visited the school where they had heated arguments with school management, they said.
Parents alleged that the van was too small and students were "stuffed in it like animals" and there were loopholes on the maintenance front.