When I was in the US I only said it when I was at one private school, none of the public schools ever did it to my memory.
I guess what could be counted as similar at my current school (In Australia) is that in assembly, the chaplain reads a name from the book of old students who have died in wars, then the headmaster says "At the going down of the sun, and in the morning, we will remember them. Lest we forget". It's for a seemingly different purpose though.
The only time we ever have a moment of silence is on ANZAC day and on Remembrance day.
