Observe the teacher in the classroom if you have a chance to. Often, though, schools do not allow parents to regularly watch teachers. Talk with your child about the classroom atmosphere and the teaching methods. Talk with other children in the class or with parents of other children in the class and compare opinions.
Discuss your concerns with the teacher before you go to the department or to the principal or to the school district. The problem may simply be that the teacher is unaware that his teaching is not being received by students. Proceed further only if the teacher refuses to acknowledge concerns or if he does not attempt to make any changes.
Present your concerns to the school's principal if the teacher is not receptive. The most effective approach will be to have several parents or students also present their concerns at the same time so that the principal understands that this is not an issue of one parent complaining about his or her kid's grade.
Step back and allow the principal to decide what to do with the teacher for the time being.
Wait for the time period you consider appropriate to see if your talk with the principal has any effect. If it does not, you may consider taking your concerns to the school board or the district supervisor.