We aren't told how those non-students gained entrance, if there were any safety precautions in place, or if they had guns in their possession. However, it's easy to imagine the horrible irony that might have followed if they did have guns: the violence in the cafeteria would have exceeded any that occurred in the gun protests around country. If there had been guns involved, these four juvenile non-students at Joppatowne would in all likelihood have been labeled as "troubled" or mentally ill.
In fact, what really prevented another potential school shooting was the absence of guns. Harford County had it wrong: what reduces gun violence is reduction in the number and type of guns. Student protests against guns supported this and should have been encouraged.