This should have been disclosed before purchasing, i am sure the seller's were aware.
School boundary changes are normally not an over night matter they are planned and approved years in advance.
Seller and agent can be held liable.
I would explain to the buyer when they are looking for homes, especially where there is new construction, that the school zoning could change. I know that can be upsetting, but it comes with the territory of buying in areas experiencing a lot of growth and new construction.