Being Bipolar and 20 years old i know that, you will have to say it very sensitively.
i havent been to college for two days and my step dad telling me that you cant just not go, just made me want to hit him!!!
although my mum stressing how important it is didnt help either.
You need to give her a reason to go, like why her life will be better if she passes the course, EG if she wanted to be a nurse, watch programs about nursing, casually say, "Oh that will be you one day!"
anything that will make her feel pressured isnt going to help.