So in a nutshell

I guess what Im saying is that your teen needs to learn how to filter. What is important to know, and what is not. Study the stuff that goes in the important to know file.
This is honestly the best exam study tip I have. I have others too in the Exam Survival Package .
If you want your teen to really nail their exam study, help them make an important/not important list.
Get a piece of paper, and make a list of what your teen would be foolish not to study.
Now your teen has a table of contents. Something to help guide their study to help them stay on track.
So simple, yet soooooooo effective.