The explosion in the Guangxi Zhuang Autonomous Region in the south, occurred as a motorcycle passed the school gate while children were arriving for classes, China's official Xinhua news agency said.
"First, the motorcycle caught fire and then exploded with a tremendous noise that could be heard from far, far away," a witness told Xinhua.
The cause of the explosion was not clear.
A man and a woman were killed immediately and the injured, who include 10 students, were taken to hospital, Xinhua said.
Pictures posted online showed the mangled remains of a tricycle on the ground and a motorcycle in flames outside the school, while several people sat on the ground bleeding.
A local official in Lingchaun County, where the explosion occurred, confirmed the toll to AFP without offering further details.
China has been hit by a number of attacks on schoolchildren in recent years, including a spate of five incidents in 2010 which killed 15 children and two adults and wounded more than 80.
Most such incidents involve knife attacks. But a man injured himself when he set off a homemade bomb at Beijing's international airport in July, in a protest at police brutality. Reports said that a 2005 police beating had left him paralyzed.
