One thing that works and doesn't involve cheating (if you have a legacy or are just against it in general) is to go to Edit Town mode. Enter the Edit Town mode, saving when you do, and click on the active house. Separate the 'problem' sims (those who can't go to boarding school who are at boarding school, or a parent who is teaching a child to walk or talk or go potty and got stuck) and move them to the clipboard. Once they are on the clipboard merge them back with their family in your active house. Return to gave SAVING AGAIN and your boarding school kids will be removed from boarding school, at home, and attending the local school. You can sign them back up to go back to boarding school after they come home. Hope this helps!