As stated above, the Supreme Court has ruled the police have no obligation to protect individuals. Basically, the police enforce laws and investigate crimes. Many police departments have the motto "To Protect and Serve", but the motto can only be enforced as a policy and not as a law. Should an officer fail to protect an individual (e.g. Scott Peterson), then a suspension or a complete dismissal would be the most likely punishment for failing to follow policy.
The Warren v District of Columbia (1981) ruling stated it is a "fundamental principle of American law that a government and its agents are under no general duty to provide public services, such as police protection, to any individual citizen." Because of this ruling and many similar rulings, I determined long ago that I was the only person responsible for my protection. I respect the police and appreciate their hard work, but I will not depend on them to protect me or my family during a violent situation. I own a firearm and I carry concealed every day. Also, I practice regularly and have taken multiple defensive handgun training courses in order to be better prepared. What I have learned from my training is that a gunfight is brutal and deadly fast, and I pray that I'm never involved in one. However, just because I choose to carry does not mean you should depend on me to protect you or your family--that's your job. Do not think calling 911 will save you. If a shooter is two minutes from your location, and the police are five minutes away, you lose. Many will say Scott Peterson is a coward for not immediately rushing into the school to protect the children. It is possible Scott was terrified, or his hesitation may have been due to a lack of confidence is his level of training. I do not know what was in Scott's mind at the time, but I suspect he was hoping for another officer to arrive in order to have backup. Clearing a building is a slow methodical process, and it is very difficult to perform alone. Before condemning Scott Peterson, maybe we should review the Broward County Sheriff's Office training regarding an active shooter.
If teachers are allowed to carry at school, I doubt they will be expected to clear rooms hunting for an active shooter. Most likely, the armed teacher will be expected to secure the room and protect those students in their immediate care.