You’re almost certain to get two answers if you ask any student why they’re not doing something that would benefit them, whether it’s taking more rigorous (or simply more) classes, getting an internship on the side, working out, joining an organization, etc.: “I don’t have enough time” or “I’m not smart enough”.
The first answer is almost always preposterous. School is the easiest, least busy time in your life, with so much free time most kids don’t even know what to do with it all. (See e.g. Jersey Shore’s ratings.)
The second answer is almost as preposterous. Just look at everyone who has come before you, and everyone who currently works in any field: are they all smarter than you? Of course not. Pick any field you want: there are hundreds of thousands, if not millions, of people working in it. Do you really think you’re incapable of doing something millions of other people have figured out?
Bullshit. You’re not too busy and you’re not too dumb. The real answer is you’re either lazy, or scared. Once you own up to it, life gets a lot easier.
(And it should go without saying, this doesn’t apply just to students)