Clearly you understand that this initially started out as you helping him get on his feet sober and get through school. But it has become manipulation, bullying, and now blackmail.
This must stop!!
Go to the school administrator, go to the guidance office, go the dean of the school. Go to anyone associated with the school who will listen. Tell them how you were trying to help but are now being pressured and threatened to continue to do his work.
This kind of bullying occurs at all levels of schooling (actually happened to me in 5th grade if you can believe that). It's something schools of all kinds are aware of and are supposed to help students break free from. I can see any school looking to suspend you as a victim of this type of abuse.
Once you have pursued help from the school and have gotten guidance on how to proceed, then I would discuss it with your mom and tell her how this has affected you. Let her lend emotional support as I am sure this will rock your relationship with BF (which should be over and done in my opinion) and having a shoulder to lean on is going to help so much.
I offer this with concern and support, but it is only my opinion and you are free to take it as you wish, okay..no judgements from me, only concern.
Hugs,
Alice