Roshan Lama, a Class VI student of Sarvodaya Government School, is battling for life at the ICU of the AIIMS Trauma Centre.
"The head injury is severe and his condition is critical. He is on ventilator," doctors said.
Meanwhile, the class 12 student who threw the javelin which injured Lama on Wednesday, was apprehended last evening.
He was produced before Juvenile Justice Board which sent him to remand home, said a police officer.
The physical training instructor of the school, Sushil Kumar Sharma and guest teacher Jeewesh Chawla have already been arrested.