Ideally, this would not be an issue. However, if a hypothetical attacker is presenting guns and other weapons that are possible to be used at a distance, it only makes sense to have the defenders using distance weapons. Tasers might also be useful, but what if the attacker has a finger on the trigger? The last thing you would want to do is send a jolt of electricity through him.
Have you ever shot or for that matter held a firearm? I have no idea what you mean by "misfire" this isn't a 18lb cannon on Admiral Nelson's Flagship. A misfire means you pull the trigger and no bang happens as your primer, powder, malfunctioned to some degree--not remotely dangerous. Secondly, REALLY? NO ONE is for giving guns to individuals who are not properly trained to use them proficiently--least of all in a school--what this question presupposes is that the individual(s) at the school who will possess the fire arm(s) will be "professionally" trained as a requirement for the permit enabling them to possess the weapon in a school zone. I was in the Marines for a grip and have done firing range training with LAPD, SFPD, SDPD and the SWAT elements of the aforementioned forces and let me tell you--if it comes down to it and someone needs to take a shot you want me taking it--not the "professionals" because their marksmanship is Heinous--even their SWAT even this SWAT "snipers" --have such piss poor accuracy, its more dangerous that they discharge their weapon then not.
School safety is more dependent on keeping the staff aware of the situation and well coordinated. Giving teachers or staff members guns would probably do more harm than good, especially in cases of misfire or those who are not well equipped for handling a gun. It is better to keep the guns for the professionals.
Arming teachers and school staff is the not way to ensure the safety of students and staff. Arming people is a reactionary response, not a preventative response. School violence and facility security needs to be addressed by preventive planning, the implementation of security and emergency protocols.
In the vast majority of US schools there is no screening of visitors and no limitation on access to the building. Addressing these issues is the way to improve school safety.
There is a number of ways to keep a place safe, and a gun is not needed to do so. While guns do have their place, I do not believe a school is one of them. If kids get caught in the crossfire more tragedy will happen then good. Tasers, or a well knowledged security officer can keep our schools safe.