A staff trustee is elected by the school staff at the time of the parent trustee elections in election year, and holds the position until the following election. For a number of reasons, e.g. if he or she gains a new position at another school, the staff trustee may resign earlier. The by-election process is described in the Returning Officers' Handbook (pdf).
The role of the staff trustee is no different than that of any other trustee and they work in the overall interests of the school. They are not the delegate for staff.