The militants lobbed the grenade at the complex of Little Mastern English School in interior Samurou village at around 9.30 PM last night, they said.
Police today defused the grenade, sources said, adding that this was the second time the militants hurled the grenade at the school after May last month.
Earlier, after the May incident, school authorities had appealed to the people not to disturb the academic atmosphere in the school complex.
Police posts have also been asked to step up patrolling and checking of passers-by to intercept the movement of the armed ultras within their jurisdiction, sources said.