I wouldn't discount depression. It can manifest itself in many ways that don't involve moping around the house.
I would echo this. I was very depressed in high school, but I never let my parents see it. I somehow escaped with a 2.9 gpa, but I don't think I really earned anywhere close to that.
My dad tried
everything to get me to study, including having me write down my homework assignments and having it signed by the teacher. If my head had been in the right place I probably would've been fine. My sister was a straight A student and went to the University of Chicago.
I failed out of college in a year and a half.