The reason for keeping weapons such as knives out of schools (or anywhere else) is to reduce the chance of fights escalating and becoming deadly. It has nothing to do with the lesser problem of killing sprees.
I believe that in the State of Texas, you can be 18 to purchase, but the firearm must stay at the residence. Meaning you can't carry until the age of 21; which means you're an adult. Besides, even with the school ban in place, there's no reason why teachers and staff can't carry anyways.