Some middle school children become sidetracked by their hormones or by sports. Other children who find schoolwork difficult would simply rather play. If parents help these children cut back other activities to reasonable amounts and count on the teacher to grade the child's efforts on schoolwork and homework, most of these children will improve. When parents over respond to this behavior and exert pressure for better performance, they can start a power struggle around schoolwork. The child sees the parents' pressure as a threat to his independence.
More pressure brings more resistance.
Poor grades become the child's best way of proving that he is independent of his parents and that he can't he pushed. Good evidence for this is the child does worse in the areas where he receives the most help. Clarify that completing and turning in homework is between your child and the teacher. Remember that the purpose of homework is to teach your child to work on his own. Don't ask your child if he has any homework.