I don't know... the teenager/parent relationship is a tricky one and, if you are not careful, can shape your relationship with them into adulthood...
My advice is to give them space to make their own decisions and even their own mistakes (providing they don't get them or you into physical/financial danger) and respect them as a young person. If you don't show some respect for your feelings, how will they keep respect for yours?
Maybe I am feeling this way because of recent problems with my own mother (I am 32, not a teen!). She has been trying to push me around recently, making assumptions that things will be ok without asking, having no respect for my emotions (regarding a family death) and refusing to negotiate a "compromise" on various issues. She is also being hypocritical and applying emotional blackmail like a professional... To be perfectly honest, I feel like a teen again! But the difference is I live in my own house, with my husband and am completely independent of my mother. So at least I don't have to put up with the treatment, but I wouldn't say it is doing our relationship any good at all... and it is quite unlikely that I will now see her over Christmas (mainly because she wont compromise - not because of me).
Somehow our relationaship has reverted to when I was a teen - that isn't right: it isn't a nice relationship and leaves everyone unhappy (my mother will be unhappy because she can't get me to do what she wants to, I am unhappy because I hate someone trying to push me around and emotionally blackmail me). I wouldn't want anyone else to have that relationship with their parent or their child.
So regarding the GCSEs, I would suggest saying something like "Ok, but when you decide you want to talk about, we would love to hear about your choices

" and leave it at that: show interest and support, but don't be pushy. Somehow the dynamic of the relationship with your offspring needs to grow with them - otherwise you could end up with a bad or "distant" relationship with them as an adult.
Just an alternative opinion
Boo