Imrpoving your written French can make a big difference to your overall understanding of French language and ultimately your grade, but identifying ways to improve can be difficult, particularly if you're looking to get into the A/A* region. These are a few ways in which you can really improve your written work.
Be studious with checking your work, particularly making sure verb endings and the gender of words and descriptions are accurate. Accuracy makes your work appear more fluent and ensures you're not losing marks due to simple errors!
Another tip for imrpoving the quality of your written French is to link your ideas and sentences with linking words such as 'cependent' or 'aussi'. Using connectives helps your work to flow better and allows you to construct more complex sentences.
In addition, trying to vary your tenses and the form of the verbs you're using (i.e don't just use 'je' or 'on' throughout) demonstrates a good understanding of grammar and helps you to avoid repetition.