They might think of better things than rocks, and it would probably have to be elaborated or combined with other procedures and devices. But the basic principle holds good. What can, with a fairly modest chance of producing what it is intended to prevent, be placed in the hands of a large number of untrained people? Worried by possible abuse, perhaps in parts of the prairie states where rocks are hard to capture in the wild? Spray the rocks with stamp-pad ink.
It isn't simply a matter of what happens once the gunman or gunperson enters a given classroom. It is a matter of not knowing what may come out of any door behind his back. A considerable number of spree killings lasted quite a while, and the perpetrator knowing he had to be in and out in a minute would have made a considerable difference to the death toll.
We also have to consider the perpetrator's attitude of mind. These actions appear to be an assertion of fragile self-esteem. Many commit suicide, or do it in circumstances where death at the hands of the police is just about inevitable. That just makes them feel heroic at last. They hate popular and successful kids more than anybody. But waking up in a secure ward, and seeing the hero who stopped him on TV, giving the audience a stamp-pad blue high-five? Getting a sports scholarship for it, like as not? It just takes seeing that a time or two to deter a lot of potential school shooters.
Another worthwhile precaution would be having a closet in every classroom with a six-foot length of 1½in. oak dowel and a heavy-duty bulletproof jacket, the kind you aren't meant to walk about in all day. You can't make every teacher carry a gun, but you can make them all have a closet, even if they will never open it, even if they just came out of one. Again, the shooter can't afford to turn his back on a doorway, and just imagine being in court in plaster, saying "I was doing fine until Miss Meek hit me with a stick."
Just wait till the next policeman shoots someone with a rock or a stick, and you will see plenty of people on the internet insisting that it is a deadly weapon, and the man with a gun can't afford to wait a second.