Doctors in the eastern state of Bihar are already pointing the finger at chemicals found in the pesticides used on wheat and rice in the area.
But authorities haven't ruled out deliberate poisoning.
"We feel that some kind of insecticide was either accidentally or intentionally mixed in the food, but that will be clear through investigations," said R.K. Singh, medical superintendent at the children's hospital in the state capital Patna, according to Reuters.
The children became sick Tuesday after eating the rice and lentils prepared in the school's kitchen. The school stopped serving the food when kids began vomiting.
Distraught relatives and neighbors have lashed out following the deaths, with four police cars set ablaze during the protests.
India's free school lunch program is one of the most important sources of nutrition in a country still grappling with poverty and hunger. According to the BBC, 120 million students across India recieve the free lunches, but critics argue that the program is plagued by corruption and substandard ingredients.
Reuters India says the cook at the school in Chapra district had complained to the headmistress about the look and smell of the cooking oil provided for the meal, but that the headmistress inisited she make the lentils and rice. The headmistress has since fled, according to Reuters, and the cook was hospitalized.
Dozens of children remain hospitalized, and authorities have said they have no way of knowing if more will die. But all the sickened children are being treated for the pesticide poisoning.