What are your thoughts on this as a way to our children while at school?

I like where this is going in that it is considering things/weapons other than guns.
I have been told more than once, get a canister of wasp spray....shoots about 50 feet, and no body messes with you once that is in their face.
How about that for "a good guy with a gun?"

Way less effective than a locked door.

There are a couple second and third order effects here.

The actual requirements for fire extinguishers are not 1/classroom. Below is the actual OSHA code, but the gist is 75' or 3000sqft of classroom/office/hallway.

https://www.osha.gov/SLTC/etools/evacuation/portable_placement.html

Cost is another concern. A singular 20lb fire extinguisher runs about $160.00 (I have 4 of them in my shop). They require brackets or enclosures, which are infrastructure costs (1 time). But they also have maintenance cost, of inspection (1/year) that runs about $10-25 per. Assume you have 25 students per classroom. My HS had 3000~ students, that would 120 classrooms + halls. Let's call it 150 to keep the math easy at $160 each + $10 maintenance, and ignore the infrastructure as our "bulk discount" and we get $25,500 per school or 3000 students. Actual cost ends up about $8.50 per student as opposed to $1.00.

Wow. A better idea would be if a shooter walked into the school he got shot in the face...with a gun instead of sprayed with a fire extinguisher.

Something needs to be done.
I honestly think the first step should be to authorize conceal/carry in the schools, publicize that you have, and make a potential shooter try to guess who may have a gun.
I bet many would think twice and try to find a softer target.

As part of the military, our initial reaction in this type of situation is to take action. Unfortunately, not everyone has that same reaction. We can give teachers the tools they need all day long, but if they don't have the initiatve to take action in a situation it will all be for naught. A lot of people would freeze in fear in this type of situation, but it couldn't hurt to try.

If you get that close, you may need to double tap him with the extinguisher just to make sure, across the bridge of the nose or over mouth will cause more pain

Probably better than cowering in a corner, but bullets are faster and more authoritative than potassium bicarbonate and CO2.

Aside from ineffective, some of the delinquent kids in school these days would probably fire off the extinguishers at some point, IMO.

The problem is that fire extinguishers are no where near as effective as the article describes. I recall having fire extinguisher fights for fun as a young Ammo troop (I blame it on poor supervision) and not once was anyone either hurt or incapacitated.

I should say that we only used damaged fire extinguishers in need of maintenance or recharging.