In a word, NO. Being active in the teacher's union and having the union present mention the person's name in speech -- without more -- would not by itself elevate that person to the status of being a public figure.
A “public figure” generally refers to either a public official or any other person who is pervasively involved in public affairs, including, for example, celebrities, business leaders, movie stars and sports heroes.
A “limited purpose public figure” refers to persons who have thrust themselves to the forefront of particular public controversies in order to influence the resolution of the issues involved. A "particularized determination" is required to decide whether a person is a limited purpose public figure, which can be variously interpreted.
Of course, a person can become an "involuntary public figure" as the result of publicity, even though that person did not want or invite the public attention. For example, people accused of high profile crimes may be unable to pursue actions for defamation
even after their innocence is established.
A person can also become a "limited public figure" by engaging in actions which generate publicity within a narrow area of interest.
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