刻苦

He is remarkably assiduous at/in his study. / He is exceedingly diligent at his lessons.

He is quite intelligent but does not work hard enough.

hard-working/diligent student

study assiduously

study hard; study with diligence

bear hardships and work hard; work hard without complaint; be willing to go through hardships

plug away at one's lessons

lead a simple and frugal life; live in frugality