Why would the inventor of Toaster Strudels need to put his daughter in public school? Surely the resident Rich Bitch would be just one of many in some ritzy private academy?
Plenty of kids with rich parents go to public school. It might be because the parents do want to save money, (even if you're rich, a good private school is still expensive) though sometimes the kid goes to public school for the general environment. More exposure to different types of people and whatnot.
In some areas of the country, public school is just as good as (and just as expensive as, due to a combination of taxes and books) private school.
Why does Cady speak English with a perfect American accent when she's spent her entire life in Africa?
Cady didn't spend her whole life in Africa, just all of her School years. She could have been five or six when she when over, so english isn't a problem. As for the accent, she could have picked that up from her parents.