In its first report in an Improving Achievement series of publications, HMI describes a "very mixed and quite complex picture" of continuing success in the early years of primary and at Standard grade, but notable weaknesses from P5 to S2. Performance in S5 and S6 is "fair but improving".
Most pupils grasp the basic literacy skills they need to deal with everyday class work but lack the more challenging aspects of language such as analysing the meaning, purpose and impact of texts.
Inspectors say that pupils are not always able to check and improve their writing, although they may have "well-chosen vocabulary and bright ideas".
By the time they reach the middle years of secondary, around a third of pupils who have done well in class tests cannot replicate their success in external exams. HMI states: "When required to work independently, their skills were not secure enough for them to achieve good results."
Inspectors highlight significant underachievement in last year's Higher English and communication exam where a quarter of pupils who gained a Credit the previous year failed to achieve an A-C pass.
A significant number of pupils are presented for National Qualification courses above their level of competence in reading and extended writing.
Eleven per cent of students failed to gain any award after moving beyond Standard grade.
Graham Donaldson, senior chief inspector, said at the launch of the document: "The intensity of what is required in fifth year to move on from Credit to Higher is significant and demanding. When youngsters are doing Standard grade in S3 and S4 and likely to go on to Higher it is important teachers do not teach just to the Standard grade but anticipate some of the more demanding language challenges around in fifth year. That is one of the areas we could do more in."
Inspectors accept that the text-sending generation do not read widely and say that students should establish good reading habits and be committed to their next stage of study at Higher which involves more independence.
In the early years of primary, HMI says that teachers are doing sterling basic literacy work, yet it tapers off around P5 and by P6 and P7 there are problems with reading and writing.
The inspectorate will now join the Scottish Executive and Learning and Teaching Scotland in running a major national conference in June on improving language across the curriculum.
A programme of staff development, concentrating on P5-S2, will follow.
"Improving Achievement in English Language in Primary and Secondary Schools" is based on HMI evidence between 1998 and 2002, Scottish Qualifications Authority data, 5-14 test results, Assessment of Achievement Programme surveys and the Programme for International Student Assessment.
HOW TO IMPROVE
Communication difficulties are said to impact on confidence, motivation and wider achievement across the curriculum.
* Evidence showed that successful courses are built around the four skills of listening, talking, reading and writing.
* Teaching needs to be clear and challenging and to promote extended dialogue with pupils so that they know how to improve.
* Problems need to be identified as early as possible.
* Courses need to anticipate the next stage.
* Language is not the sole preserve of English teachers. Successful schools see language development as a collective responsibility.