Isn't Araragi explicitly not a good student? In episode 3 he talked about how he didn't care at all about his grades as long as he could graduate, and was alienated from his parents by not keeping up the "studious son" image in high school. Maybe he's just self-deprecating, but I got the impression he was an average student, or more likely the kind of student who could do better if he tried but has no motivation.
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