Is this King's College School? What subjects are you doing?
Back in my day (~5 years ago) , it was three exams (maths, english, and something like the Oxford TSA), and then a bunch of interviews (I remember the general, maths, and chemistry ones vividly, and I'm assuming there must have been a physics one too). It clearly didn't go too badly for me, as I got offered an academic scholarship! (Hence why I'm posting anonymously, as I don't want to be too identifiable...)
Advice: look up the examination format, make sure you know what you'll be doing, and revise that GCSE content for the subjects you'll be examined on. Basically just that, really (know your stuff). For interviews - be alert, interested, and prepare basic questions such as "why King's / what do you think about IB / what are your interests", and just be yourself, really.