Once again, the argument focuses on the relatively few (thank the Lord) school shootings and protecting the children from "madmen" (What madmen? The parents? The teachers? The homicidal janitor?) and not on something simple like a licensed carrier dropping off or picking up their children, or a teacher who is working late and is concerned about his or her personal safety.
If you must argue the public school massacres, the "gun-free" zones don't do anything about the typical perpetrators of the school violence: students who are already prohibited from carrying weapons without adult supervision in most states. They're already breaking the law; the gun-free zones just add another law to break. Some of these killers had already murdered someone before they even came to school! The Columbine shooters had already violated a raft of federal laws before entering into the building. For whatever reason, these young killers already clearly demonstrated that all the laws in the world won't deter them from their vicious goals. What the law does do is essentially eliminate the possibility that someone might be able to stop them before the body count gets too high.
The security guards in my local school district not only do not carry sidearms, they are prohibited from carrying them. They can't even carry a concealed weapon with a CHL, thanks to a brilliant piece of state legislation. Armed officers are available, courtesy of the constable's office, but they are too few to be posted in even the high schools on a regular basis. Some districts do have commissioned police officers who do carry sidearms, but you might have one in a school; there don't seem to be enough to have one in every school.
Furthermore, when the first law enforcement officers do arrive, they generally delay entering the building (this isn't a criticism: there are very good reasons for this). Only someone on the inside has the chance to end the killing spree.
Nobody is protecting the children by stripping those who would normally protect them of the means to do so. Teachers have already given their lives trying to protect their students.
This isn't just a matter of a federal law; it's a matter of well-intentioned but misguided legislation at the state level and it needs to be addressed.