Police said Murugesan (49) died shortly after he was admitted to the Kovilpatti government hospital.
Initial investigation revealed that Mohan, who was dismissed from the school on charges of sexually harassing the headmistress, his brother and another person carried out the murder as revenge.
Murugesan had sacked Mohan after holding an inquiry on the orders of the Madras High Court following a complaint from the headmistress.
However, Mohan thought that Murugesan was responsible for his dismissal and hatched a conspiracy to murder him. Further investigation was on, police said.