A classroom message board could be an excellent tool for the teacher to assess the class’s knowledge of the subject. While I have never personally been a part of a class that a message board, I can certainly see the benefits of it.
The teacher could pose a question to the class that can cause a class discussion/debate on the board. This would allow each student to see what their peers believe and respond in full response, without being cut off by anyone. This also allows those that may not be willing to speak up in class a chance for their opinions to be heard.
I think the best type of class for this to be used in would be History, because each student could bring perspective and can do extra research to prove a point or to bring up a new one. However, I think math could be the worst for this because it is so matter of fact, if you know the answer you know the answer, there is not much background information out there to improve understanding like there is in History or a Science.