I've never heard of anyone being prosecuted in such a case for violating a gun free school zone.
That said, once you get to your car your choice to return to a dangerous area, much less to use deadly force against another is going to hang on lawful defense of a third party and unless your wife or child happens to be pinned down under fire, you are not likely to have any special relationship or responsibility to offer defense. Again, I've never heard of anyone be prosecuted for violating the gun free school zone in such cases. Indeed, most often you hear NOTHING about them at all as I'm sure the media and other gun grabbers would just as soon nobody realize that an armed private citizen helped end the attack.
Of course, the question remains how long you will be without a concealed weapons permit rather than being able to legally carry while on campus.
Bottom line, IMO, once you make the decision to intervene with deadly force, carry laws of any kind are going to be among the least of your concerns. If you've acted otherwise legally and morally appropriately, not likely anyone is going to charge you. If you screw up and act when you should not have, the low level misdemeanor for violating carry laws will be the least of your legal concerns.