As stated many times. A good guy with a gun will always stop a bad guy with a gun.It's not fan fiction.
Why don't you go talk to your local school liaison police officer and find out. You know, in person. You're not going to find a 30 year old former combat veteran who is on the SWAT team, but really likes working with kids. You're going to find a middle aged mother of 2 who can't handle shiftwork and needs a day job.
What's fan fiction is people's belief that these police officers who work with the schools have extensive training and practice regularly for active shooter scenarios and have the correct tactical mindset.
Armed guard resigns after it emerges he 'hid outside' during Florida school shooting (yahoo.com)