How can schools recruit, and keep, more teachers?
What's the biggest issue facing England's schools?
In the last few weeks you'll have heard that it's funding, or the kind of school, or what subjects pupils study.
You could argue for each of them, but in the end it's what happens in the classroom that matters most.
So perhaps we should be asking instead - does the teacher know their subject inside out, and are they experts in passing on their knowledge?
England has a teacher shortage and it could be one of the biggest headaches facing a new education secretary.
For several years now, not enough people have started teacher training.
At the same time a baby boom has been working its way through primary schools and is now hitting secondary schools.
By 2024, that means there will be 8% more pupils in primary schools, and 20% more secondary pupils, compared to 2015.
Last year, the target number for graduates starting training as a secondary teacher was missed by 18% overall.
And there's another problem too: growing numbers of young teachers are leaving within a few years.
Maths and science teachers are among the most likely to leave, which is why they are headhunted by secondary schools already.
That's despite the £25,000 to £30,000 you can get as a bursary to train as a teacher in these shortage subjects.
More maths and physics lessons than five years ago are being taught by someone who hasn't studied the subject beyond an A-level themselves.
So what's behind this?
Like the rest of the public sector, teachers have now had years of pay restraint, which has led to pay falling behind other graduate jobs.
A mid-point graduate starting salary outside London was estimated in 2015 to be £28,000 whereas a teacher starting out now could expect £22,244.
In other words, a social worker, quantity surveyor or IT analyst for business could expect a slightly better starting salary outside London.
Economists say the 1% cap on public sector pay should be looked at again if teacher shortages continue.
There's no sign of reassessing the cap in the Conservative plans - but Labour and the Lib Dems say they'd allow pay rises to go above 1% - keeping up with rising living costs.
But it's not just about pay.
Despite an effort by ministers to look at ways of reducing their workload, it remains an issue.
Teachers want more time outside the classroom to do their job properly.
Compared to the most successful education systems in the world, they get less time to prepare or to study themselves to become better teachers.