Joint Commissioner of Police (crime) Vishal Garg said that two physical training (PT) instructors of the school were arrested after interrogating them.
The parents of the girl child lodged a complaint at the Jadavpur Police station, a senior Kolkata Police officer said.
The child after returning home yesterday from her school - G D Birla Centre for Education - kept on crying and complained of severe pain. Her mother spotted blood stains on her clothes, he said.
Her parents then took her to a paediatrician who advised her to be taken to the police, Garg said.
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The girl was admitted to the state-run SSKM Hospital late last night and tests were performed to confirm the alleged sexual assault on her, hospital sources said.
Her condition was now stable and she was released, the sources said, adding that the investigation report has been submitted to the police.
Following the incident, parents of the students were staging a dharna in front of the school for around 15 hours demanding immediate suspension of the accused teacher and action against the Principal S Nath for alleged inaction.
Meanwhile, the G D Birla Centre for Education said in a statement to the media that the school's management was extending full cooperation with the police in their investigations and will continue to do so.
"The school management will take necessary and proper action after the investigations are over," the statement said.
Narrating the sequence of events, it said, "The principal had received a call around 10:45 pm yesterday from the officer-in-charge of Jadavpur Police Station inquiring about the whereabouts of a particular teacher.
"When she enquired about the background for seeking such information, she was informed that the parents of a Lower Infant student of the school had lodged a molestation complaint," it said.
It was found that the girl had left the school with her mother at 4 PM on the day after school hours and the school has started further internal enquiry, it said.
The statement said representatives of the West Bengal Board of Secondary Education have also been apprised of the matter.
West Bengal Education Minister Partha Chatterjee said he is looking into the incident with "utmost seriousness".
"I have no words to condemn the incident. Strongest punishment should be given to those responsible for the incident," he said.
Women and Child Development and Social Welfare Minister Sasi Panja said the child welfare committee has been asked to visit the school and the hospital where the girl was admitted.
"I just cannot think of what has happened in that school. The authorities of the school had assured three years ago that they will ensure safety in the school premises," she said.
West Bengal Commission for the Protection of Child Rights, Chairperson, Ananya Chakraborty said, "I have asked the school authorities why CCTVs were not installed there in the last three years, but they have no answer."