NZ Writers' College principal Nichola Meyer said changes in language had been affecting children's writing skills for years.
"The language they are hearing is all jargon. There is a lot of slang and it's almost phonetically based and not spelling based.
"So when they have to sit down and write something, it is completely alien to them."
The fact that English was a second language to many students was also a big factor, Ms Meyer said, as other cultures were traditionally verbal and therefore reading and writing were not the norm.
Reading had also become less of a hobby to children and more of a chore.
"One of the biggest determinants for a child doing well in reading and writing is the number of books in the home - not just magazines, but shelves full of books - and weekly visits to the library," she said.
"Reading and writing are all inter-related."
The results, based on figures from almost 2000 schools, are not "moderated" - checked for accuracy and consistency between schools.
Many teachers and educational experts have criticised the system, which they claim is unreliable, narrowly focused and likely to lead to an unhealthy culture of competition among schools.
The figures show boys are doing worse than girls, with 61 per cent of boys achieving at or above the standard in writing, compared to 75 per cent of girls.
The statistics were even worse for Maori and Pasifika students and pupils from low-decile schools.
In writing, 58 per cent of Maori and 54 per cent of Pasifika students achieved at or above the standard.
Figures sent to the Herald on Sunday show students from poorer schools did not do as well as those from rich schools.
Fifty-six per cent of students from decile-1 schools achieved the reading standard, 50 per cent writing and 52 per cent maths.
Higher-decile schools achieved scores around the mid-80 per cent mark in all areas.
Professor Judy Parr, head of the Auckland University School of Curriculum and Pedagogy, said writing was the "poor relation" of reading, and much more could be done to improve results.
She said reading was something that needed to be encouraged among children, as it played a significant factor in a child's ability to write.
"Reading is fantastic. It gives you the notions and schema of structure, it develops vocabulary.
"Any child who is driven to write will write. It could be story-writing, but not all children like writing stories. They could describe how you make something, write in detail how something works - all that helps develop better writing."
Education Minister Hekia Parata said recovery programmes existed for students who were not achieving in maths, writing and reading.
"We want those students who are doing well to do better and we need to do different things to raise the achievement of those who are falling behind," she said.
"We need to understand precisely where and how we fine-tune our education system."
