Due diligence in the hiring phase. Background checks. Psychological profiling tests. Simple observation. Last but not least, real action on the part of administrators, when red flags are noticed.
Now, little of that will honestly be implemented until parents start screaming bloody murder. Until they do, these sickos are just going to keep doing this.
Considering the size of our country, these incidents are pretty rare, 1 or 2 every few years. They get so much exposure, especially with the internet, that when it does happen, it seems likee it happens all the time and is at epidemin proportions. I can’t see anything like what you suggest actually getting implemented.