Authorities have called for nationwide enforcement of food standards in schools amid an ongoing investigation into a Shanghai-based food supplier accused of serving low-quality lunches with expired ingredients at a private school in the city.

The Shanghai Municipal Food and Drug Administration confirmed in a statement on Tuesday that students at the SMIC Private School in the Zhangjiang Hi-Tech Park were served expired food, including rotten and moldy produce. Expiration dates on the packaging of some products had been altered.

The investigation confirmed what the students' parents reported during a surprise inspection of the school's kitchen on Friday.

The State Administration for Market Regulation and the Ministry of Education urged local education and food authorities on Tuesday to strengthen inspections of campus canteens to ensure their safety.

On Saturday, SMIC Private School apologized to students, staff and parents and promised to take full responsibility for failing to supervise the vendor, according to its website. The principal of the school has been dismissed.

At the same time, the municipal administration ordered all kindergartens, primary schools and secondary schools in the city to examine their food quality immediately. It carried out unannounced inspections of kitchens at 28 other schools in cooperation with Shanghai Eurest Food Technologies Services, the supplier, as well as of the company's warehouse activities in the city.

Members of parent-teacher associations, together with food quality authorities, conducted surprise spot checks in school kitchens on Sunday. The thorough investigation revealed that a private kindergarten and an international school cooperating with the company had kept expired goods in the kitchen, although no food safety problems were exposed in the other schools or the company's logistics warehouse.

The Market Supervision Administration in Shanghai has ordered the three schools and kindergarten with the food safety issues to end their relationship with Eurest. It helped them select new food suppliers and prepare new lunch plans for students.

An investigation is continuing, the statement said.

Eurest is a subsidiary of Compass Group UK, one of the world's largest food providers for schools. It has supplied the private school for four years.

An international catering giant, Compass Group UK entered the Chinese market in the 1990s. According to research by Zhiyan Consulting, a domestic enterprise for industry analysis, Compass is ranked ninth among the top 10 catering providers in China.

In response to the scandal, national food security and education authorities urged local government officials to improve school food safety supervision and set up a corresponding long-term mechanism to tackle problems, which may include weekly inspections by parents and school leaders, along with unannounced spot checks.

"The campaign to eliminate any problems caused by unhealthy goods and unhygienic conditions requires supervision by government and schools and careful observation by parents," said Wu Chao, the father of a 7-year-old boy.

"It also involves a matter of morality, beyond the legislation. We trust kindergartens and schools to take care of our children, and hope they are trustworthy enough to offer a safe and delightful learning environment."