Are custody and visitation set via court order (including consent order) or unincorporated separation agreement? If via court order, your husband’s ex will likely be in contempt if she fails to abide by currently-in-effect court-ordered visitation schedule. If via agreement, it’s not enforceable and the child’s mother can take the child anywhere she wants and put her in whatever school she wants. Don’t waste your time pondering the “what ifs” of the agreement.
You have two choices: you either come to new agreement with child’s mother, or you go to court and have judge decide. You can also ask the judge to prevent the move and/or set new custody and visitation arrangement. Good luck!