Forty-five children were hospitalized on Friday after a mass outbreak of food poisoning at a private school in Shanghai.

The students were all from Tongyuan Experimental Primary School in the city's Jingan district.

Officials at the local center for disease control and prevention said 10 of the 13 students chosen at random on Saturday tested positive for norovirus, the most common cause of acute gastroenteritis.

None of the children were in critical condition, according to the school's director of student affairs, who asked to be identified only as Zhang.

"Some of the students have recovered, but others still have minor symptoms, such as stomach cramps and nausea," she said on Monday. "The CDC suggested the students return to school after 72 hours when their symptoms should have disappeared."

Zhang said the CDC had taken samples from the school's canteen for testing. The results, which will be available in two weeks, should determine the cause of the food poisoning, she added.