High school. And if I seriously thought that most a-students retained that specific knowledge or even those skills throughout adulthood, I might agree.

At any rate, if your kid isn't motivated enough to try for those grades now, you can't force that motivation. (And that's assuming that you aren't wrong in thinking they can just get better grades by trying harder. It's surprisingly common for people to call teens "lazy" when really they have one or more of: a learning disability, multiple learning disabilities, a mental illness, multiple mental illnesses, some sort of underlying physical illness that isn't diagnosed, low-grade school refusal due to a bad social environment, literally any other condition that's not laziness or a bad attitude or a bad character or otherwise a failure of willpower.