District Collector J K Jain ordered an inquiry into the incident.
"Fifty students were taken to the district hospital after they complained about uneasiness. They were sent home after medical check-up and being provided medication. We have evacuated the school," Mr Jain said after visiting the spot along with Superintendent of Police Gaurav Tiwari.
"We have ordered an inquiry into the incident. The police is questioning the owner of the cold storage," the collector said.
The students had assembled on the school campus for the daily prayer when the ammonia gas leaked from a pipeline at around 10.15 am. After the leak, some students complained of uneasiness, he said.
The ammonia gas leakage caused an explosion of a cylinder in the cold storage, Mr Jain said.
SP Tiwari said teams of fire brigade and ambulances were rushed to the spot after the incident was reported.
"The situation is now under control and we are questioning the cold storage owner," he said.