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.
Police have begun a probe into the incident.
All four injured admitted in a hospital were said to be out of danger.