All my kids did great until high school, when they diverged. Three of them continued to get really good grades, and the youngest, who I never ever had to ask about homework was salutatorian of his high school, with only a single A- marring his record. One of the others, however, started to spiral his junior year due to other factors, and by the time he was a senior it was just like you're hoping, just to get good enough grades to graduate. There was a lot of looking the other way by teachers who liked him, but he probably should have flunked out if we're just talking about effort.

FWIW, I wasn't a terrible student, but I think I graduated with something like a 3.5 or 3.6 and pretty much none of the badgering, threatening, or bribing my parents did had any effect. You're either motivated, or you're not.