Saran District Magistrate Abhijit Sinha said 10 children died after the mid-day meal at Dharamsati primary school in Masrakh in Saran, about 100 km from here.
More than 60 children fell ill and were admitted to hospital for treatment, he added.
State Education Minister P.K. Shahi told media persons that preliminary investigation suggest that oil used to cook the meal was contaminated. "But everything will be clear only after the probe," he said.
Angry over the deaths, hundreds of people took to the streets in protest and surrounded the local police station demanding action against school teachers and officials responsible.
Nitish Kumar expressed shock over the incident and announced a compensation of Rs.2 lakh to the kin of each of the dead school children. He also summoned Shahi to discuss the issue.
The oppposition Bharatiya Janata Party has demanded resignation of Nitish Kumar on moral grounds, while Rashtriya Janata Dal chief Lalu Prasad blamed the deaths on rampant corruption in welfare schemes.
In Bihar, widespread corruption is reported in the mid-day meal scheme and government guidelines on food quality are ignored.
In past, dead lizards, frogs, insects and a rat were found in the food cooked for the mid-day meal, angering students and parents.