About 50 children were taken ill after drinking water from a handpump at a school in Bihar's Sitamarhi district, say officials.
On Thursday, 30 children had fallen sick after drinking water from handpumps at two separate schools of Saran and Gopalganj districts.
The Friday incident took place at the government middle school in Runnisaidpur of Sitamarhi, a district police officer said.
The sick children were first admitted to a primary health centre (PHC) in Runnisaidpur and later over a dozen, who were critically ill, were shifted to the district hospital for treatment.
The children complained of pain in the stomach and started vomiting after drinking the water.
"It appears to be the handiwork of some anti-social elements as the handpump might have been poisoned," education department officials said.
After the news spread, hundreds of villagers thronged the PHC where the children had been admitted.
Expressing concern over the incidents, Chief Minister Nitish Kumar said: "I am really worried as after the contaminated mid-day meal tragedy, now reports of poison in handpumps at schools are coming in from various places."
The chief minister added that state Director General of Police Abhayanand had been directed to initiate a probe into the incidents.
As many as 23 children lost their lives July 16 after consuming contaminated mid-day meal at the government primary school in Dahrmasati Gandawan village at Mashrakh block of Saran district, 60 km from Patna.
The mid-day meal scheme is running in 72,000 schools across the state with 1.60 crore students availing hot, freshly cooked meals.