It is unwise to make a complaint if you are still at the stage of reacting emotionally: calm down first. Then be sure you have your facts right and don’t just accept your child’s version of events as the truth. Most school complaints procedures encourage dealing with concerns promptly and appropriately but the context of the complaint will determine what happens next. If possible, ask to meet the teacher to discuss your concerns and do your best not to sound accusatory. Listen to what the teacher says and only take your complaint further if you are not satisfied with the response.
If the matter relates to a Safeguarding or Child Protection issue you should speak immediately to the school’s designated Safeguarding lead. The school’s Child Protection Policy which should be available on its website.