Having done my GCSE and A Level speaking exams not too long ago, I remember the feeling of sheer panic! I was terrified and nervous but I survived it. The trick is to try your very best to stay calm and collected. When you panic, your conjugations and pronunciation can go out the window!
In more practical, "French" terms here are my three key tips:
Learn some set phrases that you can rely on for all situations. "Nous vivons a l'heure où" will never not impress an examiner!
Although your accent is important, for GCSE it's not the be all and end all! What your saying and your accuracy is far more important and earns you the marks!
Put a "donc" on it! (Not to be confused with The Blackout Crew's hit single, Put a Donk on it!) By using donc (therefore), you're adding explanation and rationale to a sentence. No one cares if you work part time on the weekend if that's all you're going to say! Make sure you explain and give your thoughts, feelings, consequences and all that jazz!
Finally good luck for the exam and try not to panic at the surprise question! Cool, calm and collected is the order of the day!