The Importance of "Demonstrated Interest"
If you didn't see it already, check out this article in the New York Times. It explains well why "demonstrated interest" is one of the new buzzwords in college applications. (And it also reaffirms why I make my students work so hard on their "Why this college?" essays!)
The article also does a great job of showing that there's no real benefit to applying to more than seven or so schools. If you do your research and choose your school well, it will all work out. Don't kill yourself on essays or spend a fortune on applications to schools whose acceptance rate hovers at 10%. Learn to write better (and I'd love to help) and enjoy your senior year!