As a sixth former who didn't do anywhere near as well in his GCSE's as he should have, I want to say this to all that read this thread, in the hope at least one takes my advice.
The mocks you take in school and the way they mark in school is way more harsh than the real exams.
Put the effort in, just an extra 20 minutes before bed, or a little less time on the xbox, and you will be setting yourself up for life. No amount of "yeah it will be alright" or "screw it" or "I don't care" will console you on results day.
People say GCSE's are easy because they are. Keep focused and calm for just another month or so, and you will reap the rewards! Good luck :P