For the "Where do you go to school" I've always said we homeschool and left it at that. For the "why" questions I've been using "it's a good fit for us."
It's vague enough that it doesn't insult anyone (we're also in a wonderful school district) and allows them to be follow up with a more specific question if they really want to know. Sometimes that goes in the "he's a quirky kid" direction and sometimes in the "we can really work at his pace this way"
Since you're just starting, you could try "we're going to see how this goes" -- it doesn't actually answer the question, but it continues the conversation in a non-confrontational direction.
