Increasingly, teaching is attracting people from other professions who are looking for more rewarding roles. This is great news for schools, which can only benefit from the broader knowledge of teachers with experience outside the classroom. Although the skills they bring from other jobs are good for pupils, people switching to teaching must do so with their eyes wide open.

If they don’t know what they are letting themselves in for, then no one gains.

At the moment, too many thirtysomething and fortysomething student teachers see their initial bushy-tailed enthusiasm turn into disappointment, even regret. They soon come to realise that the wholly rosy perception of teaching, as promoted by the Government in recruitment adverts, is a misleading one.

They find out, for example, that teaching is, in practice, no more of a “people job” than any other. Yes, standing in front of 30 children imparting knowledge is exhilarating, but no one should kid themselves that they can have a close individual relationship with each pupil. And there is absolutely nothing more solitary - and potentially depressing - than sitting at home late at night marking last week’s homework and planning next week’s lessons while your partner watches television alone next door.

I sometimes took my work to the pub and would sit nursing a pint as I marked - that way it feels less lonely.

As for the long holidays, well that’s the only time to gen up on a new topic or new syllabus that will have to be taught when school starts again.

I once had to teach six different historical periods, some up to Oxbridge-entrance level, all of which were completely new to me. Without the holidays beforehand, this would have been an impossible task.

One other common mistake made by people coming to teaching as a second or third career is to assume that the pleasure their own children provide will be replicated in the classroom. But this is simply impossible in an environment where you are struggling to keep large ragtag classes, some of whom do not want to be there, on task week-in and week-out.

Even the exhilaration that comes from teaching a really great lesson can disappear as soon as you remember you have to teach a less well-prepared topic to a worse class at the other end of the school in, literally, 30 seconds’ time.

I am no longer a teacher, but my last three years in the classroom were spent as a history teacher at St Paul’s school in London which, year after year, has been the top-ranking boys’ school in the country. I have also done lengthy stints outside education, including an office job in the City of London and political research at Westminster. This has allowed me to see, first-hand, how challenging, as well as how rewarding, teaching can be. It has also shown me clearly how schools differ from other workplaces.

I picked up four specific tips that people joining teaching from other professions should remember above all.

First, don’t look as if you know everything. Schools are their own little communities with their own set rules and their own unique foibles.

Integration is never immediate for new pupils or new staff. Even if you were previously the chairman of Tesco, you would still need a friendly soul to show you how the lunch queue works, or which rooms pupils are allowed in during break, or what the normal punishment is for late attendance. The pupils will be as annoyed as your new colleagues if you pretend that you have seen it all before, and they will wait to see you fall flat on your face. They are also likely to be as interested in but less impressed by your previous position than you expect.

Second, be firm when unreasonable expectations are loaded on you. Even more than in an office, where colleagues tend to have a better idea about each others’ workloads, if you let yourself be treated like a workhorse in a school you will soon become one. Learning how to say No - or at the least “Not unless there is a worthwhile pay-off” - is paramount.

In my second year at St Paul’s, I was expected to teach more lessons per week than any other teacher as well as run extra-curricular activities four days a week. This occurred even though an inspector had said in the previous year that I should not be given any additional duties. It happened because I chickened out of refusing.

Third, management-speak is anathema in most schools. If you talk of “synergies”, “silos” and “tipping points”, other teachers will not be impressed. But - and here’s the rub - management ideas do have real value for the average classroom teacher. The job is, after all, to manage large numbers of children each and every day. Many teachers survive only by following the 80:20 rule which dictates that 80 per cent of the outcome is due to 20 per cent of the effort. So keep your management-speak to yourself. By all means, put it into effect and let the results speak for themselves.

Fourth, never, ever come between a teacher and their caffeine fix. The mortal sins of every staffroom are using someone else’s mug - if you think that’s a cliche, check out the TES website (below) - and making a drink from a coffee jar that you have not paid towards. If you fall into this trap, you will rue the day for evermore.

It is widely accepted that schools gain from having a diverse range of children with different backgrounds. Equally, they gain from having diverse staffrooms which reflect more accurately the world beyond the school gates.

So there is real benefit to be had from encouraging people to move from other professions into teaching. But there is nothing to be gained from projecting a false view of the bewildering challenges - as well as the unrivalled opportunities - that exist within the classroom.

Nicholas Hillman is research fellow at Policy Exchange, a think tankTESstaffroom forum: www.tes.co.uksectionstaffroomthread.aspx?story_id=2134729path=Opinion