What is a good punishment for bad grades?
You will get better results if you focus on the effort put into school rather than the grades. They will come along eventually. Very bright children tend to shut down when they are harped on about how good their grades should be. In the same way, children who struggle with learning shut down if they feel that they will never be able to please no matter what. It is amazing the difference you will see when you don’t even mention grades but encourage them about their effort. Find ways to reward them when they put forth good effort but withhold privileges when they are lazy and unmotivated.