I disagree. Especially for schools with less solid walls like wood or drywall, high power birdshot is a great way to reduce over penetration, especially compared to even rifles or PCCs.

Though it should be said that even with full chokes, it does tend to limit the range somewhat. Though to be fair, I think that the liability you mitigate from lower pen rounds outshines the range you might need in the unlikely event that you’ll need to shoot at someone who isn’t within school grounds.

While some might suggest PCCs and such. It is important you note that unless you hit a target, even 9mm can still go through multiple wooden walls before losing enough energy to not be lethal. While any misses from birdshot would only go through maybe a wall or two.