The teen has been sent to a Juvenile home and, over the next three days, will be questioned by the CBI from 10 am to 5 pm. The agency will continue questioning other students, say sources, adding that given the sensitivity, the case is being handled directly by CBI chief Alok Verma.

The student, reportedly under psychiatric evaluation for a year over anger issues, had been questioned several times in the past few days and his account kept changing, sources say. He was among five people seen on security footage near the toilet where Pradyuman's body was found.

The CBI said the student had planned the killing but hadn't identified his target. Pradyuman, a student of Class 2, just happened to walk into the toilet and present an opportunity.

Four students and a teacher reportedly told the CBI that they saw him with a knife a few days before the murder. The same weapon was found in a commode in the toilet, the CBI believes. It is not clear why the school didn't act and haul up the student for carrying a weapon.

Before the murder, the student had allegedly bragged to his friends that the exams would be postponed and they should not bother to study. His school record shows he was "not a very good student", officials claim.

The student's father said he was told at midnight yesterday that his son was being detained for murder. "My son did not commit the crime. He informed the gardener and teachers after finding Pradyuman's body. He stayed in the school the entire day, and appeared for the exam. There was not even a single spot of blood on my son's clothes," he said.

A schoolbus conductor seen exiting the toilet just before Pradyuman on the CCTV footage was arrested hours after the murder. The police had said that Ashok Kumar confessed to killing the boy after trying to sexually assault him; questions were then raised about an outsider accessing the students' toilet. Sources say he is not off the hook yet, but the CBI is focusing on the student for now.

CCTV footage shows Pradyuman crawling out of the bathroom after his throat had been slit. Forensic experts say his vocal cord was ruptured at the first strike and he couldn't scream. Pradyuman was seen collapsing next to a wall, his blood pooling around him. His body was discovered around 8 am, not long after his father dropped him off.

Pradyuman's father had accused the school of a cover-up and had alleged that the police had done a shoddy job. "We never believed the conductor was the killer...the sexual assault theory didn't fit the time-frame. Our trust is restored in the investigation," said Varun Thakur.