。
Studying is a student's true calling.
Literal
Study [nominalizer-のが] student [gen-の] proper-duty is.
本分 ('true duty, proper role, essence') is a specific, somewhat old-fashioned word for 'what one is fundamentally supposed to do' — stronger than 仕事 ('job') or 務め ('duty'). Carries a moral/essential weight: 'the thing that defines what it means to be a student.'