Sierra went to a very strict catholic school that she hated so the line "rethink everything heard by the careless choir" could mean that she's rethinking everything she heard at that school and instead of just accepting everything they tell her, she's deciding that she has her own beliefs and she doesn't have to conform to other people's ideas.
Just a guess, but that's what i perceived from that certain line of the song
Sierra went to a very strict catholic school that she hated so the line "rethink everything heard by the careless choir" could mean that she's rethinking everything she heard at that school and instead of just accepting everything they tell her, she's deciding that she has her own beliefs and she doesn't have to conform to other people's ideas.
Just a guess, but that's what i perceived from that certain line of the song