And how are they jamming all frequencies?

The common ones are typically designed to transmit on common frequency ranges (wifi, cellular, GPS).

What’s the range on them? I suspect by outputting a tiny amount of power, which is shit against any competent setup.

You can buy 1W-1,000W UWB jammers on alibaba. It's very unlikely that a drone the size that they're showing in the article is going to be able to transmit at a higher power. It would cause disruption for thousands of yards with a clear line of sight.

And what happens if they’re using FHSS?

Doesn't matter. These things output noise on all channels for the frequency range that they're designed for (say from 2.4Ghz to 2.5Ghz). All channels are being jammed, so jumping between them won't do much.

what I’m saying is no school shooter is gonna be as competent as a specialist in that field, and I think the company bets on that.

Oh yeah, you're probably right and, it's entirely possible that they're using non-standard frequencies or some other countermeasure. On the other hand, software defined radios exist if someone wanted to do consumer-tier electronic warfare.

It's not likely to happen, but neither is a school shooting.

If a county is going to pay millions of dollars for a system claiming to be the last line of defense between a mass shooter and school children, then it shouldn't be something that can be completely shut off with a $50 piece of equipment. Maybe I'm missing important details, but if I were looking at this project I'd have some questions about the specifics of how these drones are remotely controlled.