Panic gripped DAV school in Palampur after a blast on its staircase during the lunch break today, but no one was injured, police said.
It is yet to be ascertained whether it was a cracker explosion or some explosives were used, they said.
Staff and students ran to safety as soon as the explosion took place on the staircase to a laboratory on the second floor of the school, Palampur Deputy Superintendent of Police Rakesh Singh said.
After the explosion, some residue was found on the stairs, Principal B K Yadav said.
The students and teachers of the school were told to immediately vacate the building, a km from the main town, and rush to the field.
The residue has been sent for examination to forensic laboratory in Dharamsala, Singh said, adding a case has been registered and investigation was on.