Teachers can be accused of making "excuses" if they dare point out that a pupil has performed poorly because of their background. Worse still, they can be blamed for it. Certain politicians seem to believe that the correlation between family income and pupils' results is a deliberate plot by schools, driven by their snobbery, or inverse snobbery.
In their imaginations, teachers must conspire darkly in the staffroom. "Well, I heard Katie's mum's unemployed, so I'm giving her 30 per cent less help in my lessons." "Oh. I just hate anyone from a posh university, so I'm working round the clock to make sure none of my pupils ever go to one. Biscuit?"
But a pupil's background is not an "excuse" invented by educators. It is the overwhelming predictive factor in how they are likely to perform, vastly outweighing the quality of their school or teacher.
Two boys can attend the same good primary, sit in the same class and get the exact same total minutes of individual attention from the same good teacher. However, if one of those boys' families is below the poverty line, he will have started his first day at school with a vocabulary half a year behind the other boy. The game was rigged before they had been assigned their coat pegs.
The government has suggested that shutting the hundred or so "bad" schools with low test scores will tackle the underachievement of disadvantaged pupils. But incisive analysis by a Financial Times journalist has shown that a jagged achievement gap exists throughout the system (see cover story, pages 22-26). Even in the "good" schools, pupils from the poorest fifth of postcodes continue to perform substantially worse than their classmates.
Do not assume that academies will magically fix this, either. Those schools are often doing praiseworthy work raising the achievements of children from deprived backgrounds. But the National Audit Office discovered two years ago that academies were doing even better with pupils who were not disadvantaged. As a result, the achievement gap had widened to a worse extent in academies than comparable comprehensives, which is awkward.
Add to that the research finding that school quality makes, at most, only 10 per cent difference to a pupil's result - if anything at all - and it is a wonder more teachers do not decide the whole job is pointless and pack it in.
Yet hope remains. Some "outlier" schools are succeeding against the ridiculous odds. What they share is almost crazily high expectations for their pupils, with targets that can seem unrealistic for young people from poor homes or who do not even speak English yet.
Schools need support to follow in those outliers' footsteps. Targeted financial assistance, such as the pupil premium money, should help if used properly, but they also need genuine recognition of the odds they are battling.
So there is the twist. It is patronising and damaging for politicians to say "no excuses" to teachers and pretend that children's backgrounds are not the biggest factor, when we know they are. But if they give teachers the support and recognition they deserve, more schools will be able to break the link between background and achievement - and they will do so through sky-high expectations that send a powerful message to all their pupils: "no excuses".