As a current flying instructor I am very surprised at the original post. Your instructor/ flying school have done you a disservice putting you forward for a flight test without the required skills and knowledge.
By the time you are ready for a skills test you should have a very good idea about Weight & Balance. (Mass & Balance for whenever the authority decide to change it back again). It’s a fundamental of flight planning and you should have done a theory exam on it too. It’s really not a good situation ‘having no clue’ at this late stage. Take the advice the others have given you and give your instructor a hard time about it too.