I'm not sure I agree on responsibility. The law is probably not on my side (which means that part of the letter would certainly need changed, but it was just supposed to be an example anyway of the appropriate kind of response), but imagine a school provides a computer lab students can use during their lunch to do research. If one of the kids decides to look at porn, who is to blame, the school or the kid? I don't see any way the school should be held responsible for that.