A victim pulled the alarm.

http://www.ibtimes.com/nate-scimio-haile...m-after-1569462

For this stabbing, I'm going with good idea. As others said above, the stabber can cause more damage if everybody is not alerted about what's going on. One girl even said she thought the stabber was just wrestling/fighting with another guy in the hallway. That's something that's relatively common in most high schools.

If the alarm is pulled, everybody is on high emergency alert. So, the stabber's advantage is eliminated.
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