Is it legal for the superintendent to talk with a board member in closed-door sessions? The board member discusses the issue with all of the other members before the meeting.
The superintendent can meet behind closed doors with a board member. The superintendent can meet with any member of the public behind closed doors, and board members are members of the public.
If the matter discussed is district business that the board can act on, and the board member discusses it with each other board member in turn, a serial meeting will have been held, and that's a meeting that should have been held in public. However, I would say that the details of such situations have a bearing on whether they are entirely appropriate. It is OK for the superintendent to call or talk with (even behind closed doors) each board member individually on a topic of board business.