How do actors/actresses get to become a character on coronation st (or soaps)?

My sister is 7 and she has acting cles and does lamda exams (london academy of music and dramatic arts) our school is really good at academic and musical/arty stuff, for example diana vickers went and this girl whos currently got a 4 album deal recording contract with columbia, recording her first at the moment...anyway, she wants to get a job on coronation st and always has, mainly because our school has big contact with them, Rosie in the soap went to our school thats why she got the part. And girls are always being extras in it. I was wonderin how people actually get parts in it though?