Who can legally sigh a child's IEP when parents are divorced, separated, or never married?

Absent a court order directing otherwise, both parents have an equal right to make custodial decisions regarding the child.  This could potentially place the school in a difficult position, but ultimately they are empowered to go forward with services upon the approval of only one parent with legal custody.  Assuming you are one of these parents, you should go to court and seek a court order concerning both the issue of legal custody and specifically how future IEP's will be handled.  The child deserves to have his parents work cooperatively to resolve his educational issues.  Moreover, there are ramifications to denying child special education services that the objecting parent may not realize.  Be sure to do some research on these issues before making a final decision.   Best of luck.