Expanding your vocabulary can help you in many ways as a student. For one thing, it's likely that at some point you'll have to take the SATs, and having an abundant vocabulary to draw from will be incredibly advantageous to you there (the higher your SAT score, the more universities will sit up and take notice of you).
For another thing, it can help you to understand more complex readings. When a student has a limited vocabulary, they spend a large chunk of their "reading" time trying to understand new words via context clues. However, when you already know the words and their corresponding definitions, you're more likely to fly through books, articles, etc.
Lastly, having a broad vocabulary helps you in all avenues of life, not just within the world of academia. You can impress potential employers by using five-dollar words instead of fifty-cent ones, to paraphrase Mark Twain. You'll appear more intelligent, literate, and well-read, which is something most employers like to see.