I am an attorney with more than 25 years of experience and I look forward to providing true and correct information to you in this regard
If there are lies being told about a person with the intent of harming that person's reputation and the persons telling the lies know the lies are not true that is defamation. And so, if these teachers are continuing on this course of reputation destruction against this principal, I suggest there needs to be a letter of "cease and desist" sent to each of them
Now, the letter may be sent to them by the principal, but if it comes from a local attorney on letterhead it may have more of an affect.
The letter simply has to say that this is a demand to cease and desist any and all behaviors that are defamatory in nature because it is harming to the reputation and that if they continue the action taken will be a defamation lawsuit
The letter needs to be mailed to each person via certified mail with a return receipt requested.
Once these letters have been issued if the behaviors continue then there is a basis for a lawsuit. And I suggest if a local attorney was not consulted to write the letters then at this point one would want to be consulted.
Now, defamation lawsuits can be costly and so you will want to be sure that the persons who are being sued have assets to collect on should the suit be successful for the principal.
In other words, you want the juice to be worth the squeeze. I might add, however, that most times with a cease and desist letter the behavior will end.