Joint Commissioner of Police (crime) Vishal Garg said 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 Education Centre - 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 protested in front of the school demanding immediate suspension of the accused teacher.
"We are looking into everything. Give us some time. I'm assuring all the parents that security is the top concern for us," she said.
"We don't want this. All the children of the school are my children," she said.
West Bengal Education Minister Partha Chatterjee said that 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.
Chatterjee said that he had heard that such an incident had also taken place in that school three years ago and the school authorities should ensure security of the students.
"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. They said it will be done this month. I am not satisfied with their answer," she said.