The incident took place in Mongolpuri in north-west Delhi last evening and it came to light this morning when the parents of the victims approached police.
The girl received severe injuries and has been admitted to a government hospital.
"The victim was raped by a man inside the school campus yesterday. Her parents approached us this morning and a case has been registered. The victim could not tell us anything about the attacker.
"We are investigating the case. We are questioning the male staff, including teachers and a guard," a senior police official said.