Just skimmed through Joshua's book, double checking. He definitely went to regular school, at least from the 8th grade on, as he mentions that money raised from the berries he picked all went to school clothes.
The school he went to was co-ed, and he talks about kissing and flirting with a girl classmate he has a crush on who mocked him for his belief in God. He doesn't mention Rachel and school, but he doesn't write about her much anyway, and never by name.
I guess it's possible she was homeschooled, but that doesn't seem consistent with what Joshua wrote, or with the fact Rachel hasn't claimed her parents were sexist and robbed her of an adequate education.
ETA...thanks for info about the Christian Academy. So she was homeschool and Joshua was not. What was the age difference between the two? I think Rachel was born in 1976 or so, which means by the time she was school aged her parents had become far more conservative politically. Maybe that accounts for the difference?
Interesting, though, that the two siblings had such a vastly different experience growing up. Might help account for her hostility towards her parents.