I advise you may retain a solicitor to issue a letter to the school to demand for a reply (and asking them to remove the relevant article and/or make a declaration to clarify untruth statements so published and/or apology to you) and reserve your rights to claim damages for dafamation.
For defamation claim, the first thing you need to do is a take-down/cease-and-desist notice, that you request the principal to remove, or correct the speech in question.
Board of Trustee is definitely not the route you should take.
Once the principal fails to respond in a reasonable time frame (like 30 days), then you can initiate proceedings against the principal and the school.
Due to complications of the legal proceedings, it is best you hire a lawyer.