I'm only 24, but even I didn't go to school with locked doors and gates. What is happening to schools? What are people afraid of? Of course, it's simply another sign of the ever increasing authoritarian culture that is developing in the United States, and the obsession with culture in our society.
My school is under guard because it DOES happen. A man recently broke into a classroom and stole thousands of dollars worth of posessions and materials. I see no reason to leave doors open, especially in bad neighborhoods. Our class has thousands of dollars worth of class and student technology, so there is no reason to leave doors unlocked.
There really is no need for the classrooms to be unlocked during break and lunch times, anyway. Not to mention, who knows what kind of personal information a student or teacher could have just laying around waiting for prying eyes. I feel keeping the doors locked when the room is empty is simply the safer way to go about it.