Diligent at or diligent in?

working hard and carefully

inactivity, indifference, laziness, lethargy, neglect

I was determined to work hard for A's, and was always a diligent student at school.

A few words are: exigent, indigent, intelligent, intransigent, negligent, unintelligent

i learnt in Latin the word diligenter which means carefully, so it is to be careful. hope this helps!