Empty Seats in the Seventh Row

The seventh row in the first grade (3) class is always empty, but there is always a blue floral jacket draped over the back of the chair. The whole class said "Lin Wan asked for leave", but she was not on the seating chart and the teacher did not remember her. Transfer student Shen Zhixia wanted to find out, but found that little by little, she was falling into a secret that no one dared to mention.