One question has plagued the Schools Week office for months: how can every school leader say they are battling a recruitment crisis, and yet we can’t see any statistical evidence of it? Even worse: we can’t see any reason for it. And without that, we haven’t a hope of investigating solutions.

So, over the summer we’ve been digging as hard as we can to figure things out.

Here’s where we are:

First, the shortage isn’t showing up in data because its collection is so fragmented. The workforce census, completed each November, doesn’t show flux in the profession, and initial teacher training data is plagued by inconsistencies – for example, the inclusion (or not) of private school teachers.

Of the numbers we do have, there are signs of a problem. As revealed in Schools Week last July, the number of vacant posts in the workforce count last year increased by nearly 40 per cent. Temporary filling of posts rose by 50 per cent.

This rate of change matters. The total number of vacant or temporary positions – about 1 per cent of jobs – sounds very low. But we must remember it has doubled – and that is alarming. If people say they are already feeling the squeeze at a 1 per cent vacancy rate, we should worry about what an increase to 2 or 3 per cent would mean.

So why might a shortage be happening?

There are stock answers: teachers are fed up, a demographic dip means fewer graduates, stagnating wages make the job unattractive.

But do these bear out? The official statistics that the government use on training and retention don’t look immediately problematic. So what else can it be?

As reported on page 6, it seems that an astonishing number of British teachers are going abroad. With international schools growing in popularity and wages stagnating at home, they are taking their chances elsewhere.

Over the summer we wrote about the difficulty new teachers have to cover rent on their wages. In the south west, newly qualified teachers face losing 63 per cent of their pre-tax salaries to live in a property of average rent cost.

Add to this the problem of retentions. First, a high number of older teachers are leaving. But, as highlighted on page 7, there’s also the fact that teachers in school-led programmes have historically had higher drop-out rates over the medium term. When these alternatives trained only a small proportion of teachers, this wasn’t such a problem. In fact, routes such as TeachFirst helped to get teachers into schools amid the last shortage. But expansion of the programme, plus the introduction of School Direct, increases the issue.

Finally, there are the problems of fragmentation and cost within teacher training. It is now £9,000 to take a PGCE. School Direct requires individuals to find a school to take them on. A plethora of bursaries, grants and “support packages” exist, but each makes the picture more complicated. When people ask me “how can I become a teacher?” I barely want to begin to answer.

Teacher shortages are exhausting for schools. Classes who have endless supply teachers get into bad habits and don’t learn enough. Staff constantly called to do cover become exhausted. The word “crisis” is not used lightly.

It is for those reasons that Schools Week will continue looking into this issue. If you have any further hypotheses about its causes or any evidence that might help us better explain it, get in touch. We also have a few more hypotheses of our own that we shall write about in the coming weeks. Plus we’d like to hear from anyone with an alternative take: perhaps you think this shortage is a fabrication and something else explains people’s worries. If so, let us know.

They don’t call us “more determined than most” for nothing.