If you are unhappy with the way in which the school has dealt with your concerns, you should make a formal complaint to the head teacher. You can do this by putting your complaint to the school in writing. State clearly that this is a formal complaint.
At any stage of the dispute, you can ask for assistance from an independent mediator. This is a voluntary process in which an independent mediator works with you and the school or local authority to work together to try and resolve your disagreement.