How to revise for German GCSE speaking?

What my teacher did for us is put together a booklet of all of the possible questions we could be asked in our speaking exams, that have to do with the topics we have studied (Health & fitness, social issues, family and friends, the environment, etc). Then we write out model answers for these questions, making sure to include as many perfect paragraph points as possible - qualifiers, intensifiers, 3 tenses, negatives, connectives, and time phrases. Then we check the grammar and spelling with her, and then learn them. This is something that should be done over the course of the subject or like over the christmas holiday, as it's time consuming but well worth it in terms of how prepared it leaves you.
Since you have your speaking mock on friday then I'd say the best preparation you can do now is to go over vocab lists on the topics you will be tested on and just practice at trying to use those words in sentences. Make sure you got word order and grammar knuckled down too, as there's no point in knowing a bunch of words but being unsure of how to string them together. Because a lot of it is spontaneity you really just have to be really confident in the basics, and be able to speak on command. Also learning the correct pronunciations is key and is what will make you sound more natural - so focus on that rather than trying to make things too complex and end up mumbling. Viel Glück