Begin the letter by typing your address without your name. Skip a space, and type the full date. Skip another space and type the principal's name and title, her school and the school's address.
Skip another space, and type "Dear Mr./Ms. (Name)" followed by a colon.
Open the letter by explaining that a situation occurred with your child and that it needs to be remedied. Be specific about the incident and what you would like to happen as a result. Direct requests usually get better results than indirect ones, so do not be afraid to ask for what you want in a firm but polite manner.
Write the details of the incident in subsequent paragraphs. Keep the tone professional, but be firm throughout that this incident or policy is unacceptable.
Close the letter respectfully, and sign your name.
Make copies of the letter in case further incidents occur and you need a legal record of your correspondence.
Mail the letter by certified mail so that you have confirmation that it was received.