A few weeks of training on a handgun or basic rifle is way more than enough. You can get reasonably proficient in basic use in a matter of 8 hours with the right instructor. Guns aren’t complicated to use.
And it takes 10,000 hours to become an expert at anything, so most police never get close to actually being expert. They just muddle through.
A lot of police never have to fire their weapon at all.
Plus, we aren’t asking teachers to DO THE JOB that police do, most of which is to interact with fellow citizens. It is rare that the police know they are confronting an armed criminal, and when they think they are, they have shown that they mostly just pull out their guns and shoot wildly.
The teachers will only use their weapons if there already is someone shooting in a school. At that point, the decision-making is pretty simple.