The 17-year-old student allegedly hit the teacher, Ravinder, multiple times with the sickle when he was checking papers in a classroom on October 12, Additional SP (Bahadurgarh), Himanshu Garg said.
The gory episode was captured on the CCTV camera installed in the classroom, Garg said, adding that the teacher suffered multiple head injuries.
The student had to be restrained by another teacher. The victim was rushed to a nearby hospital, where his condition is stated to be critical, the police official said.
Out of fear, the student allegedly attacked his teacher, the official said.
Garg said the police has taken the attacker and his friend into custody. "The student's friend was apprehended for supplying the weapon."
The attacker has been charged with attempted murder, he added.