I've posted about this before, but it's extremely difficult to find just the right person to be an SRO. And you do need just the right person to do the job right. A good SRO is worth far more than their weight in gold.
I've been teaching nearly 20 years, always with SROs, maybe four or five of them total? Only one of those was excellent, but she was really excellent. Of the rest, I did get the feeling they'd be willing to go engage a shooter, at least, though their job performance the other 99.99999999% of the time wasn't always what the school needed.