Non-residential parent attempting to enroll child in different school district

I am the residential parent of my children, according to the court memorandum until our trial and final divorce. The mother has the children 2 days a week, and her parents watch the kids (day care) during the week (at the same place the mother lives). With kindergarten approaching I've just found out she signed our son up to go to a different elementary school (district) from where I live. I've signed him up for the correct school district, however she has the kids over night on the night before school and will likely try and take him to the school she wants him to go to. What can I do?

Additional info: The mother lives in a "gray area" where one of several districts can be chosen. One of the options is the district that I live in. In other words, if our son goes to the school the mother chose, only she is in the school district. If our son goes to the school in my district then we both are in the school district. There has been no final parenting plan established, and no one is listed as "custodial" parent, however I am listed as "residential parent" and the mother is listed as having "visitation."