Exams are not designed to trick your teen, and examiners are not expecting them to know everything there is to know.
Your teen will be examined on the topics that come up repeatedly in class the ones that they have spent multiple lessons going over.
Your teens biology teacher might have mentioned some interesting/completely left field fact about whale sharks last August. Is this going to be in the exam? Heck no. But your teen better understand the difference between meiosis and mitosis, otherwise theres no point in turning up to the exam.