The teacher suffered injuries and was admitted to a government hospital at Chengalpet, they said.
The boy was expelled from the school at Sogandy last year for alleged misconduct based on the recommendation made by the teacher, school authorities said.
He attacked the teacher with a sickle on the school premises, a school official told PTI.
The boy tried to escape but was caught and handed over to police.
"The teacher has been admitted to hospital. She needs to undergo some minor procedure in left eyebrow. I was told that she may get discharged in a day or two," the official said.
Police said they have registered a case under various IPC offences.
Since the boy was a minor, he would be sent to a juvenile home, a police official said.
The police have seized the sickle and the motorcycle in which the boy had come to the school, he added.
This is the second such campus crime in the state in the past few weeks.
On September 26, a 45-year-old woman professor was allegedly stabbed by a guest lecturer on the campus of Madurai Kamaraj University.
The guest lecturer was apparently upset over non-renewal of his service contract.