Answer:
Dear (your principal's name), I need to inform you that I have caused damage to a book that the school owns. I have regretfully looked over my decision, and am very sorry for what has happened to it. Please be informed that this may not happen again. If it does I take full responsibility for the mistake.
Step-by-step explanation:
It is always a good idea to make your text sound formal, especially when you are writing to someone of high authority.