I mean, considering the cops are just going to play on their phones and actively stop people from trying to rescue children (Uvalde was more fucked up than most people remember), it makes sense to consider alternatives.

In all seriousness, this is actually a ridiculously hard problem and a horrible "solution". Drone weapons work best against unaware/unattentive enemies in a target rich environment. While a bunch of russians are goofing off and not even bothering to close the hatch on their tanks you fly forward with a couple pounds of explosives duct taped to the drone and detonate when you get in range.

Against a shooter who is already having an endorphin high from murdering a bunch of kids and is on high alert for if/when the cops will stop their fun? They'll hear the motors echoing through the hallways even louder than the kindergarteners bleeding out and crying for their parents who have been handcuffed for daring to try to help. They'll shoot more kids in the time it takes the drone to get to them and children gasping for breath will likely die as their airways close up from the pepper spray bombs. Or the drone will hit a kid anyway since aiming those at high speed needs VERY good reflexes and skills.

Again, we all know the actual way to reduce the number of children murdered in school shootings. But clearly The Second Amendment is more important than the death of a few thousand kids.