Not that this is a regular occurrence but my marks in high school averaged at 50%-60% up until grade 12 where I gave a **** and took chem, bio, physics, geo/algebra, calculus and data management. My average went up to 91% that year. On the flip side, a friend of mine in the gifted program isn't doing so well life now. I admit both of us are anomalies.
Keep in mind that there are students who go home to living hell with an alcoholic father and/or sexual abuse; they will often flunk and turn to other things like drugs, pimps, and other stuff. This isn't about being politically correct; it's about not being a giant dick. Teachers are supposed to be role models, they should be leading by example especially when the parents aren't.