Good, sensible article, John. Thank you.
My understanding is that in NSW state schools at least, there is some value added information. You may be right about the quality of the data received by the secondary school when the kid first rocks up, but schools certainly know whether kids are doing better or worse than they did at primary school. As you point out, some of the so-called "best" and often most expensive schools would hate this value added data to be made public, and some of the schools teaching our most disadvantaged kids would love to see it shouted from the roof tops.
My own rule of thumb about picking a good school, of any flavour, is to look at the Principal. If they are sensible, enthusuastic, engaged and dynamic, my guess is you've picked a good school. If they are dull, boring,pompous, and guarded, run for your life.
A friend in a disadvantaged state school was teaching Advanced English, she had a class of 4 in year 10. All 4 got high band 5 or band 6 results. Her top student was (as is usual) immediately poached by the nearby private school. 2 years later the other 3 students who stayed in the State school received over 90 for Extension English in the HSC. My friend searched and searched for her ex-students name in the honour role, unlike her 3 peers, it did not appear.
Not everything that glitters is gold, it seems.

Great sensible article. There is light at the end of the tunnel when you read an article like this, at last someone is being sensible about the whole "choosing a school" argument. But you are right, unfortunately people are blinded by the OP results at the end of the schooling cycle. I often say to other parents, 'yes, that sounds great, but I wonder what primary schools most of these students came from; before they hit the secondary years. You cannot get any information on this fact. And yet the secondary school takes full credit for wonderful OP results. And interestingly enough, if you look at the year book each year in the year 12 section of the school my daughter attends, you will note by years of attendance, that 85% of the students never attended the college from Prep to Senior. Most came in the Senior intake at year 8! The comment posted by enaj is delightfully to my taste. This is exactly what I believe, my rule of thumb is the same, look at the Principal, if dull, pompous, guarded, yes definately run for your life and don't believe any of the wonderfully 'educated' interrpretations they give you to describe what the shcool is like. They will definately let you down! And be warned, what looks great on the outside can be frighteningly different once on the inside.