Grammar Girl's 101 Words Every High School Graduate Needs to Know (aff link). If you master one word per day, you'll have the whole thing nailed down in one semester.
What I really want to do is drive you over to Annette Lyon's review of this book. Here's one paragraph she wrote:
This [book is] different. If incoming college students know these words, they'll be a step ahead in cultural literacy and understanding the world around them, whether it's the news, politics and what the politicians are saying (or calling each other), technology (important in today's world), science (ditto), or even pop culture. [Emphasis added - JS]Again, go read the whole review at Annette's blog.
And if you're thinking that six bucks is a lot of money, ask five of your classmates if they'd be interested in it. If you find four others to each throw in six bucks you can get super saver shipping (what am I talking about, you already signed up for Amazon Prime for Students ((aff link)), right?). Then you all can encourage each other, compare notes, etc. as you go through the book on the same schedule.
Knowing these words, and more importantly, USING THEM CORRECTLY will put you at an advantage throughout the rest of your academic and professional careers. This is a small change that would make a big difference. Dare I call this an example of the Pareto Principle?
I don't own this book (yet). If you have it, let me know if you like it and what you got out of it.