Breathe. It'll be fine, trust me. I'm one of the new Year 11's, so hopefully I can give you some help! Firstly, you need to make sure you stay on top of your work. I did this sometimes, and everything fell into place. Other times, not so much. It was then that I ended up in detentions. One time my teacher shouted at me so much that I had a panic attack. For exams, I'd suggest revising 2 weeks before. And I mean really revising. Then you won't panic the night before! For controlled assessments, make sure you have your plans all neat and readable, and don't fret if you forget something and end up improvising. However, remember that you're not actually sitting your GCSE's until next year, all you need to now is memorise some crap. It'll be over before you know it!
One other thing: Don't stress out if you get a U on a mock. It's a mock, not the real thing.