Writing a letter to somebody cannot constitute slander. Libel and slander are two types of defamation. Libel is written defamation and slander is spoken defamation. With both libel and slander, publication of the defamatory material is required in order for somebody to have a cause of action. If you do not publish the material about the principal to other parties, there is no publication, and therefore, there is no defamation.
I believe you are asking whether you can be successfully sued for libel, which is a type of defamation claim. Slander refers only to verbal statements. Without understanding the specific facts of what you intend to write, it may be helpful for you to note that a successful libel claim involves any untrue statement(s), viewed by a third party, that causes damage to the person about whom the statement is written. The principal at your school and/or the issues you'd like to address may also be considered public in nature, which may make it more difficult for the principal to prevail in a lawsuit if one were filed. You should also note, however, that lawsuits are sometimes filed without merit. Even if you have a good defense and ultimately prevail, your defense could be expensive, time-consuming, and an overall unpleasant experience. If you intend to pursue drafting a letter, I would advise that you use caution to keep your emotions to a miniumum and stick solely to the facts, avoiding any personal attacks. You may also consider using another approach and request to meet with your principal to discuss the issues. Although I am unable to offer you specific advice without knowing more details, I hope this was helpful to you.
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