Schools are dominated by feminist leftist who hate gun ownership. The huge majority of schools will never consent to allowing CCW. It is overwhelmingly emotional. Never, never, never!!! They will not do it.
Everyone makes decisions based upon emotion and values, not reason. The purpose of reason is to justify what we have already decided based on our emotions and values.
States should pass laws requiring all schools to have armed security- which can be satisfied by private guards, cops, or CCW. Israel has a similar law for all schools with more than a hundred pupils. Schools there primarily used armed private security guards. It is quite effective against terrorism.
If it is optional, most schools will not do it.
It is not a silver bullet obviously, as there have been some bad school shootings at schools with school resource officers.
The most effective thing, in opinion, would be if it was possible to persuade media outlets to stop giving these killings publicity. The FCC could write guidelines for media, and attempt to persuade them to follow them. It couldn't be mandatory because of the first amendment, though.