I live in Pudong New Area. I want to know how to sue the school for making up lessons in winter vacation, where to send the anonymous letter, and is there any other way?

Report to the District Education Commission and any news media by anonymous letter. But I can see that you are worried about getting caught by the school, ha. The best is: 1, computer printing (handwriting will not be seen), 2, take the letter to the post office in the area far from school and home.