Improving school security with a color code?

I saw a news article about a local high school that has a new approach to security after the recent killings. They do not lock their outside doors, do not have metal detectors, and do not have a policeman at school. What they will do is give each teacher three colored cards that are red, yellow, and green. If the teacher thinks that there is an intruder in the school they will lock their classroom door, tell the kids to be quiet, and then slide a card out underneath the door. If the card is green everything is perfect in there. If it is yellow then a student is missing. If it is red they have a problem and need help.

Do you think they have been reading too many homeland security coloring book instructions or do they just not understand about intruders, guns, and potential violence? Do you think this will make the problem go away or that it will be worse since intruders will know where all the students are?