Police said the grenade was found laying in an empty ground beside the Haldibari Girls High School in Jalpaiguri district's Haldibari on November 15. It was kept in the backyard of the local police building adjacent to the ground.
The explosion happened on Monday afternoon when the bomb disposal squad was trying to defuse the grenade.
"The explosion took place in the police station premises when the members of bomb disposal squad were defusing the grenade. However, as the school is barely 100 meters away from the police station, the sound of explosion could be clearly heard from there as well," an officer from the Haldipari Police Station said.
"The students present in the school thought the blast had happened inside the school and panicked. Some of them got injured while rushing out of the school building," he said.
Panic gripped the area following the explosion as the locals rushed to the school ground to find out what had happened.
Police claimed there was no risk in defusing the explosive device there as the place is at a safe distance from the school.
"None of the girls got seriously injured in the incident. We are investigating how the grenade reached there," the officer added.