This is a question many students have. I wonder more about the reason - is it the content - are you struggling with learning the material and need help? Is it a language barrier? My first suggestion is to talk with your teacher. And, importantly, what do you mean by "can't understand the teacher"? Meet with your teacher, ask your parents to join you if you don't feel comfortable, and be very specific on what you mean by not understanding. Maybe you need a tutor to help you. Maybe you can stay after school for extra help. Maybe you can join or form a study group for the class to review materials you've studied. I always recommend a study group - you can complete homework together and study for tests together. It's always helpful to speak to other people in your class - you each have a different perspective on the material.
Most importantly, narrow down what you mean ..... be specific and then your parents and/or the teacher can help! Please try these suggestions and I hope that your understanding of the teacher and the course improves!