Pull out your tax bill and see what ISD receives your school taxes-that is where your child should attend. If your property is huge and expands over more than one district, you might get to choose, but I think it is based on what district your physical address is in. In Arkansas, we have 40 acres (purchased in 2 transactions) that fall in the different counties and in different school districts. Since we don't live there, it is not an issue of what school to attend, but we do pay school taxes in two ISD's (based on the property that falls in their district).
However, your situation may be more like others have suggested. example: you live in Arlington but pay taxes to Mansfield ISD or you live in Grand Prairie and pay taxes to Arlington ISD.