A "lack" of principals, of course.
Boom boom - there's a crowd-pleaser that never fails to entertain the staffroom.
But while it's not bad as a joke, the impending crisis of headteacher recruitment - a genuine case of lack of principals - lurches suddenly closer as the new school year hoves into view.
The National College for School Leadership says a demographic time bomb is beginning to engulf us. Here's how it described the problem in 2006: "Nearly a quarter of heads are aged over 55, and as they retire over the next five years, the profession will be deprived of a great swathe of experienced leaders. At the same time, too few new candidates are putting themselves forward for the role."
I was thinking about this last weekend when - if The Daily Telegraph is to be believed - you and I and the rest of the nation fretted about feeling we had been abandoned. With Gordon Brown holidaying in Scotland, Harriet Harman having completed her brief stint at the helm, the Telegraph reported that First Secretary of State Peter Mandelson was "running the country by BlackBerry" from his holiday in Corfu, where he has been staying at a villa owned by merchant banker Nathaniel Rothschild.
Shock horror: last weekend Britain was being run on a kind of digital auto-pilot and no one had told us. It's like being a kid again when we were promised by our parents that they would definitely be back home by 11pm and then, when they weren't, we cowered behind curtains and misinterpreted every strange noise in a once familiar house.
That revelation of abandoned Britain came - poignantly, but deliciously - on the weekend that the obituaries of Hollywood screenwriter and director John Hughes were published. One of his most memorable bits of screenwriting was Home Alone. And there we were, British citizens supposedly reduced to a nation of trembling wide-eyed Macaulay Culkins.
The Telegraph report was a reminder of the way some people expect leadership to be high-profile and visible, the need to know who's boss and where they are. It is an issue that has always interested me about schools. The best heads I have worked with have been the ones who have stalked the corridors, taken assemblies, endlessly interacted with pupils and staff.
But it might be that this is now an old-fashioned and unsustainable picture of school leadership, if the recruitment crisis is to be overcome. Some people argue that, ultimately, it is a conception of headship that is actually self-defeating, even egocentric.
They might point out that junior doctors at the local hospital probably aren't worrying unduly about whether the chief executive of the hospital trust is in the building. The cashiers in our supermarkets may not be fretting that the store manager isn't within eye contact. Should schools be any different?
As leadership guru Professor John MacBeath wrote earlier this year: "Schools don't need charismatic heads. That kind of leadership disempowers. It makes everybody else into followers. A teacher can be a leader; so can a pupil. It's about creating a culture where the youngest child or youngest teacher has scope to come up with an idea."
So it may be that in the coming year there are two things we could do to help develop the next generation of school leaders.
We could give higher priority to highlighting the attractive features, the benefits and rewards of being a school leader. Hearing about the good bits - to counter all the negative stuff that swirls around at results times (falling standards) and after Ofsted reports - might encourage staff who think it is all about crushing accountability, paperwork and conflict to see how much of the role is deeply rewarding, hugely creative and - dare I say it - fun.
Second, new models of school leadership are rejecting the idea that every school needs its own autonomous headteacher. Federations and executive and associate headships are showing that system leadership - schools working collaboratively under single leaders - is making a big impact.
So now our parents, governors, staff and others in the community need to see the benefits of this approach because it runs counter to what we used to think. Not having a head in every school - as Professor MacBeath suggests - may actually prove a liberating factor in developing the leadership of other staff and raise standards for pupils.
It means the achievement of the pupils at the school across town should count just as much to all of us as the results in our own school. And that will require a change of mindset for lots of people, not least those of us who assumed that the role of the headteacher was to stalk the corridors.
This isn't quite the same as saying you can run a country or a school from a shady terrace in the Mediterranean, but it does suggest that perhaps we need to develop a more grown-up view of what leadership actually means in practice.
- Geoff Barton, Headteacher, King Edward VI School, Bury St Edmunds, Suffolk.