Why was an armed person allowed to enter the Robb Elementary School unchallenged?

[Double Naught Spy]

Good for Oklahoma in getting their officers certified and teachers trained. Of course, there were several certified officers with active shooter training at Uvalde and the teachers had been trained as well. Arredondo, chief of the school district PD, showed up in the first few minutes of the attack. He had active shooter training as recently as last December. Of course, he showed up without a radio...because you know, he wanted both hands free to operate his gun. Uvalde's SWAT had toured all their schools a couple years prior to familiarize themselves with each facility.

https://www.texastribune.org/2022/05/27/...-timeline/
https://nypost.com/2022/05/31/pete-arred...-massacre/
https://www.vice.com/en/article/wxdwgn/u...n-facebook

All the training in the world is great until everyone screws up by the numbers.