Not every ISD has their own PD. My district is one of the largest in the state and we do not.
I don't know for sure but I'm guessing it has to do with money. Districts that get a shit load of money from Robin Hood have to spend it or lose it, so they spend it on crazy stuff. Districts that pay money into the system like mine can barely pay enough to retain good teachers. We can't afford our own police force. In fact a few years ago they cut the funding that hires a resource officer from the city PD and every middle school post their officer. They could only afford the high school cops. Then after one of the other many school shootings, I forget which one, there was community pressure to put them back in the middle schools. We still can't afford to put one in every elementary school, as there are dozens of those campuses.
And in the all the years I've taught with a resource officer on my campus, none of those officers ever inspired me with much confidence that they would much good in an emergency.
That's my guess anyway, I'm too lazy to look up where Uvalde lands in the gets vs gives money pecking order.