If I am the custodial parent, can my ex husband enroll our daughter in a school in his district?

You, as the custodial parent, can unenroll her, enroll her where she is suppose to be enrolled, put on file she is not to be unenrolled without verifying custodial status, and then advise your ex it is done.. Once you show the school the appropriate court paperwork, he won't be able to play the game twice. Fix the issue, if it is the only one, rather than getting bogged down in court over one matter with a reasonably simple fix.