Senior Superintendent of Police Manu Maharaj said four students of the Urdu Middle School in Marwari colony of Khajenkala in Bihar were injured in the blast.
"Prima facie it appears to be the work of anti-social elements. A crude bomb was kept on the roof of the school building. While playing, some students touched it triggering the explosion," SSP Maharaj said.
All four injured admitted in a hospital were said to be out of danger.
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.