The accused, a student of Class 6, was granted bail till January 30, after being presented before the Juvenile Justice Board.
The class 1 student, who is admitted in King George's Medical University (KGMU) trauma centre, was attacked with a kitchen knife by his senior on Tuesday .
The matter came to light on Wednesday after the KGMU authorities revealed about it to the media.
The police were informed about the incident by the school when the reporters reached the trauma center.
Lucknow SSP Deepak Kumar, while talking to media, said that they have sent the hair found on the body of the victim for a DNA test.
"A kitchen knife was used for the attack. We have sent the hair found on the body of the child for a DNA test.The principal has been arrested", Kumar said.
Meanwhile, Uttar Pradesh Chief Minister Yogi Adityanath also met the injured boy in the hospital on Thursday.(ANI)