Only 53%, on average, of Grade 4 maths teachers answered a simple fraction question based on the Grade 6 curriculum correctly three years ago, a study has found.

Only 72%, on average, of Grade 5 maths teachers got the same question - 1/4 + 3/5 - right in 2009.

The study was done by JET Education Services in 268 schools in eight provinces, excluding Gauteng. Researchers tracked 8383 children over three years from Grade 3 in 2007 to Grade 5 in 2009.

JET senior research fellow Dr Nick Taylor said the poor subject knowledge of teachers "must have a profound effect on children's learning".

"Teachers can't teach what they don't know," he said.

The impact of their poor knowledge is exacerbated because many pupils did not have proper access to textbooks and were "almost entirely dependent on their teachers for knowledge", Taylor said.

The study forms part of the 3Rs project during which several organisations, including the Human Sciences Research Council, researched numeracy and literacy in schools.

The study also found that the "overwhelming majority" of maths teachers avoid challenging topics - on average only 24% of topics in the maths curriculum was covered by Grades 4 and 5 teachers. Teachers cover the "simplest of topics" and most teach in a "mechanical [and] procedural fashion".

"This practice has the most disastrous effects on the mathematical knowledge to which learners are exposed."

Researchers found that extended writing exercises were done "very seldom in South African classrooms" which is "one of the biggest shortcomings of the school system".

Grade 5s in the Eastern Cape only wrote paragraphs or longer writing exercises about twice a year, while Western Cape pupils did it almost eight times a year.

If pupils do not do enough proper writing exercises in earlier grades they will struggle to analyse and interpret information later on, Taylor warned.

Pupils in 44% of Grade 4 and 32% of Grade 5 classes did not write any paragraphs in a year, which is "most disturbing".