If I were secretary of state for education, there’s a sentence I hope I would never use.

This is awkward because, in my 32 years as a teacher, 15 as headteacher, I reckon I’ve heard this sentence spoken by pretty much every secretary of state and plenty of other officials.

And I can’t criticise them: they’re saying what we probably want to hear.

And yet, I hope I’d never say it. The sentence is this: “We have the best generation of teachers ever.”

I can see why politicians, stepping into what they assume may be the bear-pit of a union conference, speaking at a governors’ event or facing a hostile media showdown, might want to use this sentence.

They’re essentially tickling our tummies and hoping we’ll purr. I can see, too, why they think it’s a sentence that parents and taxpayers generally might want to hear.

But I think it’s a sentence best avoided, because how do we know whether the current crop of teachers is the best generation ever? How would we ever know such a thing? And what does it say about those proud generations of former teachers, all of whom played their part in passing on to pupils the best that has been thought and said?

It’s a reminder of what Mark Twain may or may not have said: “All generalisations are false, including this one.”

I was thinking this the other week when meeting school and college leaders in Durham and Preston. They told me how irksome it is, how insulting, when “schools in the North” are gathered together in a single soundbite to be criticised from Westminster for not instilling aspiration, standards and rigour.

Why, the implication goes, can’t they run schools more like those in London?

These leaders resented the lazy way in which schools like theirs, often serving communities of entrenched disadvantage, are caricatured and then dismissed. They resented the way that the judgements on schools - in performance tables, in inspection - often make their job of leadership in their context so much more difficult.

In her brilliant book Making Good Progress?, former teacher Daisy Christodoulou warned us:

“One important principle is that assessments have to be selected and designed with reference to their purpose. Different assessments will serve different purposes; we cannot expect one assessment to produce all the inferences we need.”

So when did we ever think it a good idea to use tests of reading, writing and maths in Year 6 to judge not only the achievement of a child, but also the performance of the teacher, the progress of the school and the leadership capability of the head?

‘No more sweeping generalisations’

Why were we so slow to spot that a school with a high proportion of students from disadvantaged backgrounds is likely to record a lower progress score than a school in a middle-class area, for the simple and unsurprising reason that disadvantaged young people often face greater challenges in their lives?

We need to be far more wary of generalising around the effectiveness of a single teacher or the quality of the nation’s teaching workforce.

And the same goes for leadership.

In an interview in Fiona Millar’s book, The Best for My Child, one of the architects of the Coalition’s educational reforms, Sam Freedman, reflects on lessons learned from creating a free market in education. Here’s what he says:

“We were misled by how different London is to the rest of the country. You have all these great heads in London so you think they can run the system themselves. Then you go to other areas and they don’t exist. The culture is different, the people are different. It is all very well saying we can have system leaders in London but that won’t work in an area where there are no outstanding leaders to turn the system around.”

Really? How could we make a judgement like that? I meet outstanding leaders everywhere I go. I see them working in challenging contexts, in communities that lost faith in education decades ago, and staying there, relentlessly working with their staff to instil hope, ambition, pride and other qualities not easily captured in performance measures.

And the fact is that we need more people like this - teachers, teaching assistants and leaders at all levels who commit to these communities and create the same sense of collective success that a metropolitan viewpoint can assume is easy but which in reality is hard-won.

And we are far less likely to attract such people to such places if they feel that the way schools are measured and inspected and then talked about makes it so much harder.

Which is why it’s probably time to pack away the windy generalisations about the quality of teaching and leadership, to talk less, and to recognise that performance data can only ever provide a snapshot and not the full story.

Instead, we need to recognise that schools, like the country they serve, exist in many different contexts; and we need to criticise less and celebrate more the exhausting, focused, relentless effort that goes on every day in every region.

Geoff Barton is general secretary of the Association of School and College Leaders. He tweets @RealGeoffBarton