It doesn’t have to be multiple armed Delta Force types. A single retired policeman/military with a desk and monitors near the front entrance. Seems like an easy ask.
The problem with that is having armed police in schools has not shown any benefit to mitigating school shooting casualties. A study looked at over 130 school shootings pre and post Columbine and schools with armed police officers had 3x more causalties in school shootings compared to schools without an armed officer. There are countless other studies that show essentially the same thing. If anything, it's just additional motivation for some kid or adult who wants to shoot up a school to make sure they're equipped to kill as many people as they can as fast and easily as possible.
For all the legitimate mental health concerns with masking/COVID protocols for kids in schools... there are equally just as many if not more issues created for mental health, development, etc. by turning our schools into armed lock down facilities. Just about any actual study out there shows there are no benefits to having police in schools, it doesn't really make kids feel safer (nor does it prove to make it any safer), and creates a multitude of other problems with increasing school-to-prison pipelines, actually increases disciplinary issues, takes away from other areas, and god knows what else. Finding a way to prevent some 18-year-old messed up kid from getting assault rifles and body armor would be the best starting point.