Custody Change for School Grades?

One of the determining factors for a custody evaluation is school, right? I am just wondering because my stepson's mother isn't doing jack to get my once all "A" student stepson from walking the line between failing and passing throughout junior high. She doesn't make sure he's done his homework at night; he's not turning in homework. He's not studying for tests. We live 200 miles away or else we'd make sure he was doing this stuff ourselves, like we do for our other children, but we can't control what the ex does in her home. Why is she letting this bright kid fail? Why isn't she checking his homework at night? Why isn't she removing his privileges (cell phone, myspace, guitars) until he brings his grades up? We can't ground him from 200 miles away. It seems like she just doesn't care.

It bugs the heck out of me to see the potential this kid had turn to dust.