Devansh Meena, a class 1 student, fell into the septic tank around 12.30 pm at Ryan International Public School in Vasant Kunj area in south Delhi.
The boy was trapped inside the tank till the sewage workers reached there and fished him out.
"The boy was rushed to a nearby hospital where doctors declared him brought dead," said a police officer.
The officer said that the hospital authorities informed police about the incident when the boy was admitted.
Police have registered a case of negligence against the school authorities and asked the principal to join the probe.
The school authorities reportedly did not inform the student's family members about the incident for over two hours.