In suburban schools, surrounded by nice neighborhoods and middle or upper middle class demographics, I suspect that the security protocols are not followed. I know they are not at my kids' schools. Sure, the door is locked and you have to be buzzed in, but they always do. Anybody can walk up, speak into the intercom, and state that they are a parent bringing in lunch money or forgotten homework, and the door opens right away.
Two months ago, my 8-year-old niece realized in the middle of class that she left her homework at home. She got up, left the school, went home, got her homework, came back, and walked right into an unlocked side door back in to the building. She was gone 45 minutes, and nobody noticed. Because nobody cared to.