URGENT HELP to perusade mum!!!!?

hi, i go to a normal school (not private) at the moment. but i REALLY want to go to St. Mary's (Cambridge) i have asked my mum and she has said no,i hav told her about good grades and everything but she still says no! she's like, why do you want to? your not movinng school! anyway, we would have to have a scholarship. but i really need some good ideas to help persuade her! thanks in advance! :)