When I was in school, they did a perfectly acceptable thing. They had teachers/administrators walking the halls from time to time, locked doors to areas where students don't belong, and teachers gave these funny things call "hall passes."
Someone mentioned bullying, and that will never be stopped. You cannot stop bullying through more control. It won't work at all. Bullying is a part of coming up in school, and it is up to the parents of the bully and the victim to discuss ways to get through it. More often then not, you stop bullying by punching the bully square in the face. Way to stop the bully from bullying is to have good parenting.
There is a line that should not be crossed, and students should be given the right to their own privacy even within school property. Like I said, why should administrators know if a student is dropping a deuce?
Hall Passes, cameras, and adults in the halls from time to time....that is all you need.