I don't think they ever say that they are homeschooled. My understanding is that the mother just doesn't believe in them going to school & being "civilized" and she only cares about educating them in practical matters (street smarts rather than book smarts).
Ah yes right street smarts indeed, one ep shows mum taking them to the park instructing the children how to forage for food. Seriously though most shows aren't going to show the mundane things in life we do every day it's boring and wastes a lot of time, if there is it's only to chew up time and it's not a good series.
If you watch the little 'making ofs' on More 4, Caitlin Moran says most of their home schooling involved watching TV and going to the library every day. Sounds like they weren't formally educated but were encouraged to read as much as they could.
-------------------------------- "Look at banner Michael!"