Teachers need to understand boys better to help them catch up with girls in reading and writing, Estyn recommended this week.
A report from the inspectorate found that the slow rate at which boys master literacy between the ages of five and seven led to a performance gap with girls at GCSE.
Estyn said more must be learnt about boys' characteristics and behaviour to bring out the best in them and to increase their chances of doing well in other subjects.
At the end of key stage 1, Estyn said boys were about nine percentage points behind girls in English - almost three times the difference between boys' and girls' performances in maths and science.
But girls' progress in literacy has plateaued over the past five years.
The Assembly government said it was confident that the foundation phase - due to become statutory in all reception classes from September - would help address many of the report's concerns.
A spokeswoman said: "Our foundation phase places a firm emphasis on language, literacy and communication skills to support the development of children's skills in this area."
The report said a majority of teachers were doing a good job teaching English and Welsh.
It also said pupils have a better idea of their progress now teacher assessment has improved, but that primaries needed to provide a balance between structured literacy and play-based language activities to help boys match girls. Boys also needed to talk over literacy exercises with teachers more, it said.
"Best Practice in the Reading and Writing of Pupils aged Five to Seven Years" is the third in a series of papers intended to help schools improve literacy teaching.
Estyn used inspection reports from 2005 to 2008 to judge teachers' performance. There were good or better standards in pupils' reading and writing in 82 per cent of English-medium schools and 77 per cent of Welsh-medium schools.
The main problems were limited fluency and expression in reading; weaknesses in spelling, grammar, punctuation and handwriting; and a lack of extended writing skills.
But only a very few schools were not providing good quality teaching - although many were failing to challenge their brightest pupils.
Dr Bill Maxwell, the chief inspector, said: "Being able to write accurately and fluently, and with confidence and understanding, is crucial not only for achieving high standards in English and Welsh, but also for giving pupils more opportunities for success in other subjects."
The report's recommendations included focusing greater attention on boys, brighter pupils, and poor punctuation and grammar.