Speak out. Go to board meetings and voice your concerns citing specific instances where students have been advanced without learning the concepts. Request to see the school's policy and evidence supporting promotion.
Be proactive. Often the teacher is overwhelmed by needy students. Rally support for the teachers. Power is in the numbers. Go to Parent Teacher Association meetings and talk to other parents about the issue to generate ideas and identify how to get more tutors or class support for teachers with struggling students.
Work collaboratively. Meet with the teachers and administration. Insist that they give you valid reasons and evidence supporting why your child should move ahead to the next grade, despite an awful year. Disagree if you are still not convinced and bring forth evidence supporting your reasons. Stick by it.
Write an opinion piece or letter on the topic and submit to your local newspaper. If it gets published, you have the opportunity to reach a wide range of influential people who agree with you and want social promotion to end.
Empower the students. Teach your child how to reach out for additional help from the teacher. No child should move ahead without mastery of a concept. It is the student's right to learn the material, and the educators job to ensure it is taught.