Why is there a windsock on top of the Meredith high school?
 

Here's my guess: A freshman influencer on YouTube is well-compensated by entertaining subscribers with scenic views of Lake Winnipesaukee by helicopter on her way to rapelling down onto the school roof every morning set to background music of Pink Floyd - Just Another Brick in the Wall.

I always thought it was for the DART medivac helicopter as a designated landing zone for a medical evac.

Is this common for high schools?

Yes, Medflights (Dart in NH) often use schools for landing zones for they usually have large spaces, parking lots/football fields, that they can land one without obstacles around.