Dear Aurélie,
These writing exercises are a true exercise in humility. I test in the 90's for C1 grammar but I consistently fail (often get less than 50%) in the writing exercises. It took me over a year to accept this - I always would find my performance very disappointing and avoided the exercises (tip to others: don't avoid. They will improve your French, painfully and slowly in my case). The writing exercises are excellent and introduce a wide variety of expressions and of contexts, but perhaps students should be forewarned that one's grammar level on Kwiziq will be higher than one's performance on the translation/writing exercises.