Seth Godin has a great post today about why you shouldn't text while working. He's directing it at the professional world, but if you're a high school student who wants to go to college, I think it applies to you, too.
You're competing against people in a state of flow, people who are truly committed, people who care deeply about the outcome. You can't merely wing it and expect to keep up with them. Setting aside all the safety valves and pleasant distractions is the first way to send yourself the message that you're playing for keeps."