This goes out to any and all who deal with policy towards a bomb threat, and to settle the stomachs of those who hear of a bomb threat. I've noticed that in a lot of my experience in conversing with people about the bomb threats that many say that the administration is overreacting to the situation in pulling everyone out of class and essentially killing the day, by wasting everyone's time. However, whenever such an intuitive reaction is brought up someone responds with the cliché, "Oh well they have to take the necessary precautions." I'm here to tell you that evacuating 100% of the time given the conditions of a bathroom threat is beyond the necessary precautions.
I'm going to assume that if someone is going to use a bomb they either have two motivations: to fulfill their sadistic desires to kill as many people as possible or to make a statement of some kind (it is possible for these to intersect, but as you'll see it doesn't matter).
Now if they are a sadistic killer and want to kill as many people as possible, then they would not even bother to make a threat because that would be counterproductive in achieving their goal. Sure enough, in the three most recent mass killings, Columbine and the Virginia Tech Shooting, no threat was made prior to the acts of violence.
A statement by the bomber simply means that the bomber is committing such an act to communicate something, in other words, he is bombing for more than his own sadistic motivation. If the bomber is trying to make a statement, then he will still either want to kill as many people as possible in making that statement or try to kill as few people as possible (or none). In the case of the former, the same logic still applies that applied to the sadistic killer and so no threat would be made. However, if the bomber wanted to kill as few people as possible (or none), he most certainly would make a threat.
There is no reason why a killer trying to kill as many people as possible would make a threat or why a killer trying to kill as few people as possible (or none) would no make a threat. 9/11 is a perfect example as no threat was made, yet those who committed the horrific act were clearly acting beyond their own sadistic intentions.
This means that if a bomber is making a threat, he is trying to kill as few people as possible (or none). The only variable in a bomb threat left to analyze is the nature of the threat, since we already know the intention of the bomber. If a bomber is trying to kill as few people as possible, then that bomber will make sure that his threat let's those that are in danger know that they are in danger. Think of someone holding a mall full of people at ransom, it wouldn't make sense for him to make a threat that does not make it clear that he would mask his intensions because if he were not to achieve his goal of receiving the ransom money that would make his attempt at bombing pointless. The only way he could achieve receiving his ransom money is to let someone who could pay the money know of his intentions. This means that, if there is a threat, then that threat has to be strong and clear enough to let those in danger know they are in danger or else it can be assumed that the threat is not valid.
Writing a message that there is a bomb in a school on a bathroom wall is not an adequate, clear, or strong threat of any kind. It can then be assumed that such a threat should not be taken seriously. However, to react to such a threat in the same way 100% of the time would be poor judgment in either overreacting or under reacting. This is because this allows those who threaten to know the outcome of his actions, which would reinforce one's decision in making such a threat. By making the outcome of one's decision unpredictable, this decreases their confidence in such a decision and makes it less likely they will make such a decision. It would be best to evacuate from a threat of this nature 50% of the time (decided on a coin toss).
Of course, I just want to reinstate that if a threat is strong and clear, it most definitely should be taken seriously.
I also want to state one exception to the rule, if someone is so sadistic that they put a weak threat knowing that there would be no response, so that after they committed their bombing they could say "I told you so." However, such a scenario is addressed by the coin flip decision to evacuate or not. Such a sadistic person would not risk such an action in this case because they would jeopardizing their true desire to kill people and so they would resort to a guaranteed surprise bombing.