What should I do if I have to be placed in the worst two classes in the third grade?

You are very talented. Both the headmaster and the teacher believe in your ability and believe that you will bring it well. In fact, a little hard work in class is hard work, but there is no pressure. As long as you grasp it well and improve your grades a little, you will succeed. Of course, you can also try to communicate with the principal. If you attend half a good class and half a bad class, you will feel that a good class is more stressful and demanding.