Hello, I am ever so sorry for the late response..too much French revision made me neglect the right balance!
Fortunately I know of a few since the internet does give some peculiar responses. You have some really good ones...here are a few I know of:
cracher dans la soupe-to ruin the party (for some reason I always seem to remember this as it reminds me of soup)
rôtir le balai- to live the high life ( you can incorporate this in the many aspects in your writing e.g. going shopping/outings with friends)
être une girouette-to change your mind all the time
jeter l'argent par des fenêtres-to throw money out of the window
ce n'est pas la mer à boire-it's not the end of the world
I really hope this helps! You don't have to answer this if you don't feel comfortable to as it is quite confidential however do you have any predictions as to what sort or grade you think that you are going to attain/what grade you would like? Also, how are you feeling in terms of the French exam as i am quite literally panicking!