Re: hard-working student vs ?
I agree that "slow" suggests learning disabilities.
I'd say "poor student" works in the US to mean someone who didn't do well in school, suggesting at least some of the problem was lack of effort or attention. It would not suggest that teachers "rebuked" him, but it seems student-teacher interactions are quite different in other cultures.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.