Open communication is the key to resolving disagreements. You should not hesitate to tell the principal if you are unhappy or concerned about something. Talk to your child's teacher or principal and let them know exactly what is concerning you, and how you think it could be resolved.
The Principal Education Officer, Student Services (PEO, SS) is a contact available at your local regional office. They can give you advice about relevant legislation and policy, your rights, and the options available to you and your child. You can contact the PEO,SS if you are dissatisfied with the outcome reached through talking to the school.
If you want to make a formal complaint, the DET website has a detailed guide, including advice for resolving problems, advice about making complaints, and an explanation of your rights.
For more information about complaints, read the State Schools: Making a Complaint 67k.