I believe that, if the child is staying with the grandparent for the purpose of having a home, and not just attending school in district A, the resident district remains district A.  Rule below and copying Steve Crew just in case my logic is faulty.

41.51(12) School district of the child's residence. "School district of the child's residence" or "district of residence of the child" is that school district in which the parent of the individual resides, subject to the following:
a. If an eligible individual is physically present ("lives") in a district other than the district of residence of the individual's parent for a primary purpose other than school attendance, then the district of residence of the individual is the district in which the individual resides, and that district becomes responsible for providing and funding the special education and related services.
b. If an eligible individual is physically present ("lives") in a district other than the district of residence of the individual's parent solely for the purpose of school attendance, the district of residence remains that of the parent; therefore, the parent must pay tuition to the receiving district. The district of residence cannot be held responsible for tuition payment.