How old are they?
It's a fine line- nag too much and they might rebel altogether- don't nag and they will blame you if it all goes wrong.
My 2 are at uni- doing 1st year exams and masters exams this week- it never ends.
I think you need to TALK not nag- nagging is water of a teenager's back.
Better to give constructive advice:
-have you made a revision timetable?
-have you allocated time each day to every subject and worked in some breaks?
-What time do you want to eat tonight- before or after you have revised for a couple of hours/one hour etc.
My son admits now he did not work for his GCSEs at all- he got mainly 10 grade Bs but could have got As with some work.
he did some revision for his As, but not a lot. Got AAB. He only began working in his 2 nd year at uni. he is now EXTREMELY motivated and spends over 12 hours a day revising in the library at uni. it is often a case of maturity.
My daughter has always worked hard and is a perfect student- she has to work hard to pass exams,but has very clear goals - good job/nice flat/nice car- and knows the only way is to work hard.
o answer your question- in my experience nagging doesn't work, but gentle reminders do and a lot of support.