Senior students attend outdoor activities in Heilongjiang Experimental High School in Harbin, northeast China's Heilongjiang Province, May 25, 2020. Students in their final year of high school in Heilongjiang returned to campus on Monday. (Xinhua/Wang Song)

Senior students study in classroom in Heilongjiang Experimental High School in Harbin, northeast China's Heilongjiang Province, May 25, 2020. Students in their final year of high school in Heilongjiang returned to campus on Monday. (Xinhua/Wang Song)

Senior students study in classroom in Heilongjiang Experimental High School in Harbin, northeast China's Heilongjiang Province, May 25, 2020. Students in their final year of high school in Heilongjiang returned to campus on Monday. (Xinhua/Wang Song)

Senior students attend activities in Heilongjiang Experimental High School in Harbin, northeast China's Heilongjiang Province, May 25, 2020. Students in their final year of high school in Heilongjiang returned to campus on Monday. (Xinhua/Wang Song)

Senior students prepare to enter the teaching building in Heilongjiang Experimental High School in Harbin, northeast China's Heilongjiang Province, May 25, 2020. Students in their final year of high school in Heilongjiang returned to campus on Monday. (Xinhua/Wang Song)

Senior students study in classroom in Heilongjiang Experimental High School in Harbin, northeast China's Heilongjiang Province, May 25, 2020. Students in their final year of high school in Heilongjiang returned to campus on Monday. (Xinhua/Wang Song)

Senior students study in classroom in Heilongjiang Experimental High School in Harbin, northeast China's Heilongjiang Province, May 25, 2020. Students in their final year of high school in Heilongjiang returned to campus on Monday. (Xinhua/Wang Song)

A security staff member is on duty near a temperature detector at an entrance of the teaching building in Heilongjiang Experimental High School in Harbin, northeast China's Heilongjiang Province, May 25, 2020. Students in their final year of high school in Heilongjiang returned to campus on Monday. (Xinhua/Wang Song)