I believe if they own the property then the school zone issue does not matter.
Just as if you owned property within a "school zone" you have a right to carry there or allow people to carry.
I cannot cite any cases, and I probably wouldnt on here even if I knew of some. The law states you must knowingly enter a school zone. As far as I know private property is still private property, it is up to the owner to decide.
Keep in mind that I am not giving legal advice, I am just trying to answer your question to the best of my knowledge.