Authorities discovered a container of insecticide in the school's cooking area next to the vegetable oil and mustard oil, but it wasn't yet known if that container was the source, officials have said.
India's midday meal plan is one of the world's biggest school nutrition programs. State governments can decide on menus and timings of the meals, depending on local conditions and availability of food rations.
It is seen as an incentive for poor parents to send their children to school and currently covers some 120 million children across the country.
It's also part of an effort to address concerns about malnutrition, which the government says nearly half of all Indian children suffer from.
While complaints about the quality of the food served and the lack of hygiene in the program are routine, the incident in Bihar appeared to be unprecedented for the massive food program.