There’s a Gary Larson cartoon that shows a man telling off his dog. The owner is saying: “Okay, Ginger! I’ve had it! You stay out of the garbage! Understand, Ginger? Stay out of the garbage, or else!’” The image carries the caption: “What we say to dogs.”

Beneath it is the identical picture, this time with a different caption. Instead of “What we say to dogs” it’s “What they hear.” Now from the man’s mouth come the words: “Blah blah Ginger blah blah blah blah blah blah blah blah Ginger blah blah blah blah blah.”

It’s a cute reminder that sometimes the message we think we are delivering somehow fails to cut through. There’s a rhetoric; but there’s also a reality.

I thought of this when reading the brilliant new book by Rebecca Allen and Sam Sims, The Teacher Gap. It’s all about the second most serious issue currently facing the teaching profession: there simply aren’t enough teachers. (The first most serious, by the way, is the crisis in school and college funding).

Allen and Sims are experts on this topic, though they probably wish they weren’t. Because the scale of the shortfall in the number of teachers needed to educate a pupil population which is set to rise by about 500,000 over the next five or six years is, put simply, eye-watering.

As Tes recently calculated, even if no teacher left the profession between now and 2024 (and that patently isn’t going to happen) then England’s schools will need 47,000 additional teachers.

And the government’s track record in addressing this issue - one that can hardly be said to have crept up on anyone - is pretty woeful. The department has missed its target for initial teacher training recruits for secondary schools for the past five years in a row - recruiting only 80 per cent of what it said was needed last year, and failing to hit its target in all but two secondary subjects: history and PE.

School leadership is crucial

And today we learn from the teacher workforce census that at a time when we need more teachers in our schools the number has actually fallen.

This isn’t only about new teachers. It’s also that we need to stop haemorrhaging the ones we’ve got. According to today’s census, already worrying retention rates are getting even worse, with 33 per cent of teachers leaving within the first five years.

And in their book, Allen and Sims bring forensic clarity to why this is. We can call it workload, but, in fact, that’s a proxy for the real issue: the key factor in whether a teacher stays or goes is the quality of a school or college’s leadership. It’s as stark as that.

Because when it comes to expectations around workload, leaders are the gatekeepers. And similarly, we are the people who define the educational climate, the ethos which either supports teacher development or neglects it.

Which is why I thought of Gary Larson’s dog cartoon. Because Allen and Sims report an alarming mismatch between what teachers say and what leaders sometimes think is happening. For example, they say: “90 per cent of school leaders reported that their beginning teachers were given constructive, non-evaluative feedback.” But asked the same question, only 34 per cent of teachers agreed. Similarly, 82 per cent of school leaders reported providing tailored professional development for new teachers, while only 58 per cent of beginning teachers reported receiving this.

So what we think we’re doing may not be what is actually happening. And such is the frenzied life of school and college leaders - accountable for everything and reacting to whatever comes our way - that we may not notice the discrepancy between good intentions and impact.

Thus when it comes to recruitment and retention, there’s plenty that we - the teaching profession itself - must do. We need to get better at promoting teaching as a career. Every school and college website could have a link to the Get into Teaching website. We could celebrate with our pupils - all those possible teachers - why teaching is a career to be ruled in and not out. We could ensure more positive stories in the local media. We could create stronger professional development programmes for teachers at various stages of their careers. We could try to make teaching a more flexible role. And we could be bolder in cutting the time teachers spend planning, marking and filling in forms. All this would help.

But there’s something significant and urgent the government could do. Because one way we can help to fill gaps in the workforce is with teachers from abroad. But, as Tes recently revealed, between December 2017 and April 2018, around 250 teachers from outside the European Union were refused visas because of an arbitrary government-imposed cap on non-EU skilled workers.

So, at the very time we most need to be able to use every possible means to put teachers in front of classes, the immigration system is putting a blockage in the path of schools and colleges. It is self-defeating madness.

That’s why ASCL has joined other education unions in supporting the Tes #LetThemTeach campaign. Together we are calling for the teaching profession as a whole to be added to the Home Office’s “shortage occupation list”, which gives higher priority for visas. Currently, only maths, physics, computer science and Mandarin teachers are on the list, but we have long argued that there is national shortage of teachers in general.

This could be a quick and welcome policy win for the government by showing a commitment on behalf of children and parents to ensure that we are able to recruit and retain at least some of the massive army of teachers we’re going to need.

In the longer term, we need huge numbers of new teachers. In the short term there’s one easy step: #LetThemTeach.

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