How can a non custodial parent enroll kids in a school in a different state without the custodial parent being aware?

I am not sure I understand your question, as something may have been left out.  Are you saying that a putative father, with no custody order in place, has refused to return the children and has registered them in school, in another state?  If so, you need to file an emergency motion in your local J&D Court, ASAP, and try to have this matter resolved as soon as possible. Once the children have been away from you for more than 6 months, it will raise questions about where the action should be filed.  I urge you to confer with an experienced family lawyer in your area to discuss this matter right away.  Lawyers charge by the hour, therefore, you need to discuss fees and expenses before setting up a meeting.  Best of luck~