I’ve heard from a few college applicants recently who are reluctant or flat-out unwilling to make their Twitter and/or Facebook profiles private. They resent the notion that an admissions officer would take the time to do a Google search and see what the Internet turns up.
A student can debate all he wants as to whether an admissions officer should take the time to look at your online postings. But it happens, and you have to face that fact. It also happens when you apply for a job and any time you do anything remotely resembling going on a date. People seek out information about others online. When they find something, you don’t have control over how they react. You can only control what they find (and you’re able to control less and less of that every day).
If your Twitter or Facebook pages have content that you’d be just fine with your parents or your high school principal seeing, knock yourself out and go public. Otherwise, play it safe and go private. You’re almost certainly not Tweeting anything worth jeopardizing your chances of getting into college.