THE publication of study plans, student coaching courses, low security and a teacher ban on next month’s national literacy and numeracy tests have turned the tests into ”flawed instruments” for measuring school performance, NSW high school principals were warned yesterday.In a letter to the principals, the president of the NSW Secondary Principals Council, Jim McAlpine, said the teacher ban on the National Assessment Program – Literacy and Numeracy (NAPLAN) tests would cause disruptions.The tests were becoming ”flawed instruments for measuring school performance” and done ”in such hostile circumstances would lack validity”, Mr McAlpine said.”The publication of NAPLAN study guides and the creation of student coaching courses have already ensured that the tests this year will not be a real guide to student progress and performance level, and the lack of security and assurances of probity further reduce the potential for NAPLAN results to be useful to individual students and their families,” he said.The chairman of the Australian Curriculum, Assessment and Reporting Authority, Barry McGaw, said study guides and coaching courses could only help familiarise students with the test format.”Beyond that it will not help,” Professor McGaw said.