I am a high school teacher. It isn't a level playing field which is why the results for Naplan are scewed towards some schools, some socioeconomic areas.
I have no objection to running Naplan but i have strong misgivings about using the data to produce league tables. For one thing it is a snapshot and the results vary from year to year depending on how bright that group of kids in your school is. Thus trying to use it to measure the quality of the education is ridiculous-if a school ranking rises 50 spots over last year then drops again with the same teachers then the data is meaningless.
Secondly, it doesn't measure true ability. In this day there are kids who get coached for selective schools tests, kids who do tutoring every day to try to extend their ability so they can outcompete others. In class some schools teach to the test-some don't. Being familiar with the test format alone will improve results but it doesn't mean those children are any brighter than those that weren't coached-just that they were better prepared for that particular test.
Finally, the published results are standardised. They don't measure the numbers of children who perform "off the chart"-eg we had a number of students whose results were way way above where they should have been but because it was averaged out it didn't show just how well some students were doing at that school.