Approach the local Parent Teacher Association of your school system with your concerns. The PTA is your first step in addressing concerns as a parent. Many PTAs have connections with school administrators and principals that are the most appropriate figures to deal with criticism.
Use health studies and reports that detail the negative impact of heavy school bags on children to support your criticism. While it may be apparent to some, your administrators and fellow parents may not be aware of the actual effects of heavy bags especially on developing young bodies.
Propose changes to current practices that can be implemented to decrease the amount of weight a child has to carry. For example, set guidelines for the number of books a child may be required to bring home and use digital homework to avoid the use of extra notebooks and paper materials.
Keep your composure when addressing fellow parents, teachers and school administrators. While your child's health is a personal subject, you are more likely to get people to listen to you if you keep calm and avoid getting emotional. Remember that even though it may not seem like it, most administrators and school employees have your child's best interest at heart.