This has suddenly been coming up a lot in our lives from younger kids (ages 5 to 10ish). I've been answering back with something like "by being out in the world.... you've learned stuff this summer while you're on vacation, right? You can learn in lots of different places and ways"
But I'm sort of looking for a variety of more diplomatic answer that totally respects school (which, deep down, I believe is not such a good option, especially 40 hours a week for little, little kids which is the norm here). Homeschooling is pretty uncommon where we are and so it really isn't on people's radar and seems to come across as crazy to a lot of kids.
What do you say? Especially if it gets specifically into unschooling?
I keep it pretty simple with things like "we're always learning. We can't help it." Or maybe that the children learn like adults do - by reading and talking to people and watching things and going new places and so on and so forth...? That when they're really excited about something or need to know something, they go find it and you help.
You can throw in a couple of interests that have been particularly exciting. Like, my son really loves this video game Minecraft and he looks at things online about it all the time and one thing he found was an Instructable about how to make a plush doll of one of the characters. Then using some materials we had at home, he learned to sew and made his own pattern. He's not great at it yet, but it's a new skill and it's fun and creative and made him have to think about spatial relationships and construction and solve all sorts of problems.
My kids would say they homeschooled. People are more likely to accept homeschooling. What most adults question is how will they get socialization. I answer that I don't want my children to be socialized by other children. My children function in the real world with people of all ages. They participate in activities like sports and scouts with other children.
Kids that don't know my kids would usually not ask any questions. My kids' friends would want to be homeschooled too! We weren't involved in a home school group because the only group where we lived was very religious and we aren't.