I would go over the teacher's head and talk to the counselor at the school or the principal. I fully believe that all school work issues should be handled by the school. Our job is to support the school and help them to be the authority figure in their classes.
Call the school and ask the counselor if they have a program where the student stays after school a couple of days per week and during that time they have to finish ALL school work, or homework. The teacher they have this class with might not even be their regular teacher. Our girl did this last year and she hated it. Too bad. She had to stay almost an hour after school twice per week. She started doing her work at school and her grades went up.
I found quite a bit of research online, and on the dept. of Education, that shows kids who do homework actually have lower grades and lower test scores than kids who don't have extra work assigned to do at home, busy work, not work they didn't finish in class, but actual "do pages XX-XX even numbered questions and bring them back tomorrow" sort of thing.
The research showed that kids who leave school at school that got to have activities and family time after school did better. They get a mental break from school and have time to build other parts of their cognitive abilities. If you only got to do work all day then came home and continued your work for hours more, not being able to spend time with your family, not being able to go to activities such as church, a friend's birthday party, play a sport, or anything because you had to do more work, you'd start to be overwhelmed and your work would suffer.
Kids need to be kids and go outside and ride their bike, play ball, hang out with friends and family, etc....
Your teen needs to be allowed to leave school at school and to be able to finish their work at school.