To me, the #1 advice for a student aiming at a highly ranked school, is that they need to spend time understanding what they really want in a school and understanding what each university really has to offer.
“MIT” and “Harvard” are pretty much abstract concepts to most high school students. On the most part high school students have almost no clue what it means to wake up in your MIT dorm surrounded by other MIT students, trudge off to class through the snow or a northeaster, sit through a class that teaches at twice the normal speed, and work through piles of homework after class. This is a good fit for some students, but MIT and Harvard and similar schools are NOT a good fit for the majority of students, even students who think that they want to go to a school with a big name.
I know lots of MIT graduates and Stanford graduates who work alongside UNH graduates and U.Mass graduates and the latter keep up just fine (or in at least a couple of cases I know are well ahead of almost everyone).
“unfortunately you can work as hard as you want and still not get in”
This of course is very true.