S.T.
it wasn't so casually common when my boys were in public school, but what i did for anything awful that came up (like 9/11 which happened when they were 14 and 10) was to discuss it with them in relatively simple terms, ie some terrible people took it in their heads to do this horrible thing, and then follow their lead. my older liked to talk things out to a great degree, my younger at that point kept it all close to the vest, but would come at it elliptically later on.
like most things, honest and simple were the only guidelines i kept to.
khairete
S.