It's an inefficient use of resources. Since 2000 there have been about 61 children killed in school shootings. Using the same number of cops to check that parents are using child seats in their cars would probably save a lot more kids.
There are other clever things we could do to somewhat harden the schools, even like installing a few roll-down metal doors in the halls, like those you see at store entrances in malls or downtown shops for when they close up for the night. Having more tasers in schools shouldn't have a downside, and you could probably rig something similar to all the classroom doorknobs that shooters always grab and rattle, looking to enter a classroom. With webcams and computers now ubiquitous, we could also monitor schools so that administrators, teachers, and police can monitor the shooter's movements in real time, improving the effectiveness of the response, evacuation, and countermeasures.
Basically, we can guess pretty well on what a shooter or bomber is going to do, based on past profiles, and can plot and plan to confuse, impede, isolate, and delay them to some extent.