Another thing you could do is write to FRO directly indicating that the child is no longer a child of the marriage as per your agreement (provide a copy of your agreement) as she is no longer enrolled in school. The FRO will contact your ex and ask if this is the case (see News Release on Ministry of Community and Social Services website dated July 6, 2006:
Ministry of Community and Social Services - News Releases)
I think your ex would be more likely to act on a letter from FRO.