The ruling shocked the victim's father who said: "A sudden current ran through my body in disbelief. I can't believe this."
The teenager has not yet been formally charged because police were hoping he would be declared an adult so they could include him in the main trial of his five co-accused.
He does not have a lawyer and his account of what happened on December 16 is not known.
Lawyers for the five accused said they would plead not guilty and one has accused police of torturing him, his lawyer said.
Police allege the 17-year-old and five men gang-raped and beat the student on a moving bus in the capital before dumping her and a male friend in the road. The woman was so badly injured she died of massive organ failure in a Singapore hospital two weeks later.
The case has sparked national debate about crime against women, and President Pranab Mukherjee, made a call in a televised address for the country to "reset its moral compass".