I know it's easy enough for me to say 'just don't listen to them' or 'just don't do what they say', but honestly, that is the best thing you can do.
Apply yourself for the college and the course where you really want to go.
Your parents can call and apply for the sixth form they want you to go all they want, but in the end, its your decision and you simply can just not turn up to there.
I am assuming they're quite strict and this is why they're being like this, but it may help to do an extensive list of reasons why you want to go to this specific college and how it will benefit you. If they really want what is best for you, they would let you do what you really want to do.