It appeared the three-wheeled motorbike was carrying a homemade bomb that went off at 7:10 a.m. as pupils were arriving at Balijie Primary School in Guilin, a resort city in Guangxi Zhuang autonomous region, investigators said.
A man and a woman were killed in the explosion, China's Xinhua News Agency reported. The man was believed to be the bomber, The New York Times said. At least 10 children were among the injured, officials said earlier.
The explosion flung a dozen motorcycles about the crash site and blew out windows of vehicles and businesses near the school, officials said.
The blast damaged some classrooms and classes were suspended, Xinhua said.
A shop owner, identified by the surname Qin, said she was assisting a student with a purchase when the bomb went off.
"The student asked me to get the notebooks. I was just turning around when it exploded and window glass shattered everywhere," Qin said.
The state-run news service said witnesses reported seeing a man on the three-wheeled motorbike passing the school as the blast occurred.
"First, the motorcycle caught fire and then exploded with a tremendous noise that could be heard from far, far away," one witness told Xinhua.