If not in your bylaws, you are not obligated to post the positions. However, you still should. It is imperative that nobody feels there is a dictatorship, or your attendance and participation will severely drop. Fresh ideas are always great, and it also prevents burnout. Our PTO fell apart for a few years after not having a change in leadership, and nearly did again when the principal told leaders they didn't have to post the positions. All groups should have elections, and if the current leaders are re-elected, at least it was done fairly.
There's really nothing in your bylaws about elections? That's hard to believe, and if it's true you need to revise your bylaws. If somehow these terms are expiring when it isn't time for elections, then there should be a provision in your bylaws for filling vacancies -- and again, if there isn't, then you need to revise your bylaws.
If somehow it's really true that there's nothing in the bylaws governing this, then do your bylaws cite a parliamentary authority, such as Robert's Rules? If so, then consult that authority for guidance. If not, then again your bylaws need work.
If somehow there's no parliamentary authority either, then probably you are not "obligated" to post the positions. But it sure would be smart. Seems like very bad PR not to.
Questions is if people finish their two year terms and the positions are now open and the same people volunteer to take on an additional term of 2 years. Are we still obligated to post these positions. Nothing in the by-laws and I want to make sure we are being fair to school community.