Free meals are offered to impoverished students in state-run schools as part of the government welfare measures in many of India's 29 states.
"The eight children died after consuming the midday meal in their school," local Bihar state administrator Abhijit Sinha told Agence France-Presse by telephone on Tuesday.
The children, who were all below the age of 10, had been served a meal of rice and lentils cooked at the school, Sinha said.
"It is a matter of uncertainty why some of the children died while others survived after consuming the midday meal," Sinha added.
Bihar state chief minister Nitish Kumar ordered an immediate investigation into the deaths. He said a team of forensic experts was being rushed to the school to determine the cause of the deaths.
Television footage showed sick children lying on wooden tables in a state-run hospital. Anxious family members fanned the children in the monsoon heat.
Sinha said that the tragedy had occurred in a state-run school in the village of Masrakh, in Bihar's Saran district.
The 80 children taken ill were all transported to hospitals, Sinha said.
Government welfare measures
Some were being treated in state capital Patna. Others were admitted to hospital in the town of Chhapra, around 65 kilometres from Patna.
Bihar's chief minister announced compensation to the parents of the dead children.
Free meals are offered to impoverished students in state-run schools as part of the government welfare measures in many of India's 29 states.
Bihar is considered one of India's poorest states and is also the most populous in the country.
Free lunches are hugely popular with poor families and educators see the meals as a way of increasing school attendance. The midday meal scheme has been in place for decades. But children often suffer from food poisoning due to poor hygiene in school kitchens and the sometimes sub-standard quality of the food.
Food prices have soared in India over the last six years, causing increased hardship for the 455-million people estimated by the World Bank to live below the poverty line.
Data from Unicef shows that one in three malnourished children worldwide is found in India, with 47% of those aged below three underweight.
Earlier this month, India's Cabinet passed a decree to offer subsidised grains to nearly 70% of the population, or more than 800-million people. – AFP