Hi,
I've suggested some changes.
Personally, if I were a teacher writing this about a student, I'd avoid personal physical comments, eg about his smile, and I'd avoid references to his character. I'd just comment on his actual work.
Best wishes, Clive
A complacent smile is permanently resident on his face, yet he underachieves in many of his assignments. There is a noticeable between his verbal comments and written work. His comments brim with insightful observations; his written assignments reflect his lack of self-discipline, work ethics and intrinsic motivation. Doubtless, the discrepancy between his capabilities and achievements is due to a significant extent to these personal failings.
mentioned in the second half of the penultimate sentence = not necessary and sounds awful.