Why were parents and others allowed to walk onto campus if there was a safety threat?
In the event that the threat is actually on campus, it is unlikely that anyone would have been allowed on campus, and law enforcement presence would have been significant. All available law enforcement were at the scene of the incident, and as things came under control, they began to arrive at CMS. Because the campus is locked down, no school personnel could leave the building to close or lock gates because this would endanger them.
