The body of Devi Dayal bore head injuries inflicted by a blunt object, they added.
Dayal's body was found inside a room in the school at around 9 am, an officer said, adding that an empty liquor bottle, two glasses and food were also lying nearby.
There were no signs of forced entry, he said.
Dayal worked in the accounts unit of a transporter, and stayed in the school as a guard at night, the police said.
CCTV footage from around the school was being scanned, they added.