Ofsted inspectors have angered teachers by criticising them in a letter sent to pupils following an inspection.
The correspondence, to pupils at an unnamed school in the South- east, has been condemned as "unhelpful and unprofessional" by the Association of Teachers and Lecturers.
The inspectors, who must report back to pupils under Ofsted's new framework, wrote: "We do not think your teachers set you challenging enough work, and when this happens you do not learn as much as you could."
The union believes such criticism is likely to be thrown back in teachers'
faces by pupils, undermining their authority.
But David Bell, chief inspector of schools, asked if the letter was appropriate, said: "Absolutely, yes. What is the point of doing school inspection if you do not say the work that is set for these youngsters is not good enough?
"We have to say the truth. You can bet that even if we did not say it the students are saying it and the students are thinking it."
Alan Gotch, an ATL policy adviser, said he did not disagree with the principle of the letters, but the way in which they were written. He said:
"We do not feel that kind of expression is appropriate to children."
Mr Bell told MPs on the education select committee that the letters were a response to findings that pupils were often unaware of the outcome of inspections.
Not all letters written since the new inspections began in September have proved so controversial. Inspectors told pupils at Radcliffe-on-Trent junior, near Nottingham: "Your teachers teach you well and you learn a lot.
Your parents are pleased you come to this school."