My son's not old enough for school yet, but he's got to the age where people sometimes say "Does go to nursery yet?" or words to that effect. I just say "No." and offer no explanation/justification. I think some people find that a bit unnerving, but I quite like that!
Being an @rsey type, I'm likely to respond to queries like "why is he not at school" by asking if they went to school, and when they (presumably) reply "yes" responding with "and look where it got you, stacking shelves in Tesco <or whatever mundane job it is they do>, living the career dream aren't you?"
Disclaimer: I don't have anything against people working in shops/doing mundane jobs, but if people give me attitude they get it back with interest!