The attack occurred about 11.40pm. A team of forensic and bomb disposal police examined the scene on Tuesday morning.
The police said the grenade was thrown from outside the school on Narathiwat-Pattani road. The grenade hit a tree branch and dropped into a ditch along the fence. A loud explosion followed, but nobody was hurt.
The police believed the attack was intended to disrupt a ceremony on Tuesday morning to give certificates to kindergartan children who finished Level 3 of pre-elementary education. Despite the incident, the ceremony was held as scheduled under tight police security.