A teacher hands out face masks to students in Huizhi Middle School in Xi'an, northwest China's Shaanxi Province, April 7, 2020. Students in the final year of junior high schools or vocational schools returned to school on Tuesday in Shaanxi. (Xinhua/Liu Xiao)

Shenzhen in South China's Guangdong Province have reported 27 COVID-19 cases among teachers and students, causing alarm as nearly 35 million students in China have returned to schools.

Shenzhen health authority reported that 21 students and six teachers were infected with COVID-19 amid the pandemic outbreak. Of the 21 students, 10 were elementary school students, seven were middle school students, and four were college students, said Wu Bing, deputy director of the Shenzhen Health Commission, at Wednesday's press conference on pandemic prevention and control work at schools. 

To avoid possible cross infection on campus, students and faculty returning from Wuhan to Shenzhen after April 8 are required to go through a 14-day health observation and take two nucleic acid tests at an interval of seven days, according to Wu. 

Besides teachers, other school staff who may have close contact with students, such as canteen staff, dormitory workers, school doctors and security guards must also take tests before returning to school.

The government has also arranged for designated health administrators at schools to take charge of pandemic prevention and control work on campus, leading and supervising the implementation of health management, testing and information reporting of the school's high-risk groups, Wu noted. 

As the pandemic ebbs, provinces in China began to announce school start dates. 

Graduating students in middle school and high schools in Guangdong Province will start school on April 27. Elementary schools, tertiary institutions and other middle school students will gradually start classes after May 11, Yangcheng Evening News reported.

More than 35 million students have already returned to schools as of Friday, said Wang Dengfeng, an official from the Ministry of Education, at a meeting on Monday. 

Several provinces and cities, such as South China's Guangdong Province and East China's Zhejiang Province, have made it mandatory for students and teachers returning from outside the regions to undergo COVID-19 tests before resuming classes. 

School dates can also be changed if infection risks are identified in the regions. 

Northeast China's Heilongjiang Province, Liaoning Provinces, Southwest China's Sichuan Province and other places that have seen an increase of COVID-19 cases have announced a further postponement of school start dates. 

The Beijing Municipal Education Commission also required that classes or even entire schools be immediately suspended if any suspected or confirmed cases appear on campus, The Beijing News reported on April 13.