The 71-year-old was attempting to prevent six men from attacking another woman and robbing the Christian missionary school last week when she was locked in a room and raped.
But the victim is said to be more concerned that the men are forgiven and the students are safe.
Arnab Ghosh, a police superintendent, said five men had been detained for questioning.
He added: 'CCTV footage depicted six men, aged between 20 and 30, scaled the boundary wall around 11.40pm and entered the school and disconnected the telephone lines.'
The robbers then tied up and gagged a security guard before entering the nuns' room, where they found the women sleeping, police said.
When the victim, who is the oldest nun at the school, tried to stop them taking another woman into a separate room they raped her.
The men escaped with some cash, a mobile phone, a laptop and a camera. They also robbed the school's chapel and took holy items.
Footage shows books and papers strewn across the floor as the men leave the staff room.
Furious residents took to the streets in the wake of the attack, shouting slogans, demanding action and blocking off the main highway.
The incident comes at a time of heightened sensitivity over women's safety in India, which last week saw a documentary about the 2012 gang-rape of a student in Delhi being banned.