A foreign exchange student moves in with a family and she has a Texas accent and there is a girl who she lives with and becomes close friends with her and she acts all innocent and shy and in school she is like a nerd but then she gets popular and destroys her mates reputation and then ends up in a beauty pageant and then her mate reveals what she is really like

There is a film called "slap her, she's French" which sounds very similar to the one you're describing. The family don't actually realise she's from America until the end of the film though.