I think this may be a false image create by the media. The current bias in the media reporting is targeting AR type rifles and trying to make it look like they are used for everything illegal that they can be.
The latest academic study I saw says that a pistol was the most powerful weapon used in around 62% (IIRC) of all active shooter situations. Having pointed out that fact, I will also point out that each shooting is an individual event and must be considered. It is kind of like deciding to carry at all. You pray it will never be needed, you know it won't be needed most of the time, and you know that when it is needed, not having it means you will never need it again.
I would also point out that a pistol rated backpack is still way better than no protection, even if a rifle is used.
I do not particularly think the risk of school shootings is that high that I would buy a bullet resistant backpack or insert for my children, but if I did, I would look at getting the best one I could afford and that they would carry. Finding the sweet spot between weight and carrying is going to be a judgement you will need to make and hope you get it close enough if necessary.