You've ruled out body armor for schoolchildren, but still implying that average American citizens ought to wear them in their everyday lives. I'm hard-pressed to think of any situation in which a civilian would need tactical body armor unless they are looking for trouble. Gangsters and indeed mass shooters like the Uvalde guy come to mind. Bad as the gun violence situation is, America is not a warzone. We can solve this problem if we want to.
And it's the simple, common-sense stuff. Impose rigorous background checks and licensure requirements on gun ownership, like they do in Japan where gun violence is practically nil. Their success shows us what a "well-regulated militia" would look like in practice. Multiple mass shootings across the country every week obviously isn't that.
Fortifying schools the way we'd fortify federal buildings is not realistic. There are far, far too many schools across the U.S:
"There are 130,930 public and private K-12 schools in the U.S., according to 2017-18 data from the National Center for Education Statistics (NCES)."
How many officers are you proposing to station at every school? Multiply whatever that number is by 130,930.