Almost a decade's worth of research data shows high school students struggle with writing more than with reading or numeracy. But without those skills, they "struggle to show what they know, and their learning remains untapped or unseen", the report said.
The research found year 9 students in 2019 were the equivalent of five months behind the level of year 9 students in 2011. On average, one in six of those students was below the minimum standard required to succeed in their final years of school. That compared with one in 20 below the standard in reading and in numeracy.
"The minimum standard isn't high to begin with," said education consultant Peter Goss.
writing research assesses how students develop and structure a piece of writing, as well as how they structure a sentence and use punctuation, paragraphing and spelling.
The decline in writing has been more pronounced for advantaged students, whose parents are educated, than their disadvantaged peers, Dr Goss's analysis showed. Boys are twice as likely to be at or below minimum standard by year 9 than girls.
The Thematic Review of Writing, handed to NESA in mid-2018 found a focus on writing at primary level was followed by "a significant decrease in teaching writing in the early years of high school" across all three sectors.
In primary school, the class teacher teaches writing, but in high school it is shared across disciplines so no single teacher is responsible.
"It is core business in [kindergarten] to years three or four, but then you look at what the teachers self-report to us … the attention shifts away from the explicit teaching of writing," said lead author Claire Wyatt-Smith from the Australian Catholic University.
Research has shown that writing ability in year 9 is a strong indicator of success in year 12.