If a quorum of the board attends a public forum does the forum have to be posted as a board meeting?
If the meeting was advertised by the superintendent as a meeting for the superintendent to discuss and gather information from the community and the board members attended and participated only as community members, the meeting does not need to be posted even if the full board or a quorum were to attend. If, however, board members all step to the front of the room and start discussing the topic or running the meeting and asking questions for their own information, I would say your situation had crossed the line and the meeting should have been stopped.
Be advised, however, that if the Board attends the meeting to gather "information that will serve as the basis of a subsequent decision or recommendation by the governing body, the meetings law will apply." Attorney General's Public Records and Meetings Manual, p. 115.
The court of public opinion is frequently as important as a court of law. If the public assumes that this was a way to avoid posting a public meeting, or there is suspicion about how your board conducts business, or the topic of the meeting is a particularly hot one that the board will soon be addressing and voting on at a board meeting, I recommend posting before gathering. We say "When in doubt, post it."
Then your issue becomes HOW to post it. This is not a board meeting, because the superintendent cannot call a board meeting. One approach is for the superintendent to advertise the meeting in the same places that board meetings are posted and, at the bottom of the announcement, state: "A quorum of the board may be in attendance at this meeting, but members of the board will not be discussing, deliberating, or making decisions. This meeting is for the superintendent to collect information from members of the public." Then stick to it -- no board member discussion.