The common word lazy can apply here.
- : not liking or willing to act or work
He was a bad student because he was lazy.
In this context, lazy implies that he did not do his work or study.
Specifically, to suggest that a student did not want to study, you could say
: not having the mind or feelings engaged : not interested
He was a bad student because he was disinterested (in school, studies, etc). 2. apathetic
: having little or no interest or concern : indifferent
He was a bad student because he was apathetic (towards school, studies, etc).