My HS did not have one, but I understand they got one within a decade of my graduation. I think Columbine was a major impetus; the change happened about that time. The difference, of course, is that there is little risk of elementary school kids pulling a Columbine or committing some other crime requiring an armed police response, so most people probably did not see much cause for armed cops at elementary schools. Until now, when we have been pointedly reminded that if those kids present only a marginal threat, they are particularly vulnerable to threats from others.