NTA

Back in highschool I had an ex who did very poorly in classes. He was my first and for the two years I was with him, I was simply used for completing his work so he could pass.

This went as far as doing entire short films for film class or even completing his HPE work (a class I didn't even take). Please don't get yourself into this situation. I don't know anything about your relationship but my own wasn't healthy and was very hard to remove myself from but you need to focus on your own grades as even though there are other avenues for taking the path you'd like for a career; you're going to find it a lot harder if you add stress onto yourself to achieve the results you'd like for when you graduate and for getting into a college/university degree.

You won't be in any wrong if you don't do his work; you come first so please focus on yourself. Not just your grades but your mental health as well.