Dodgy labour practices bring to mind muddy building sites and gang masters, not the respectable world of British education, but the experiences of one woman - and her case is by no means unique - suggest that teachers considering taking on extra temporary responsibility (or “acting up”) should be on their guard.

Mary (not her real name) is a talented and ambitious young teacher at a school in north-east London. She had worked her way up to head of year and was then asked if she wanted to “act up” as deputy head. The deputy himself was “acting up” to replace the head who had left. Mary leapt at the chance and was offered temporary contracts, repeatedly renewed, until Christmas, when she told the school she was pregnant.

At the start of the new term in January, she was told there was no longer a deputy-head contract for her, although the real deputy is still covering the head’s post. She must continue to act up - and she is being paid for it - but she no longer has the position officially, and she has no idea how long she will be asked to continue. Mary suspects strongly that the extra responsibility will end well before she is entitled to a deputy head’s pay during her maternity leave.

Education experts believe problems such as Mary’s will grow if the shortage of school leaders continues, and more and more teachers find themselves “acting up”. Schools squeezed between a lack of senior staff and budget headaches may well find that finessing contracts is one way to trim costs, says Steve Jones, a lecturer in education who used to be a headmaster himself.

Mr Jones (not his real name), who used to teach Mary and still works in the same area of London, believes that the advent of private education consultancies, such as the one which has replaced Mary’s local education authority, has not helped.

“These private agencies know where all the labour loopholes are, and how to exploit them,” says Mr Jones. “They are out to make money. Of course health and safety, training and labour issues are going to be affected by that priority.

“I’ve worked both with LEAs and with these agencies and it’s quite an eye-opener,” he says.”

The differential between maternity benefits for someone on a simple teacher’s contract and that of a deputy head is significant enough to make the sleight of hand over Mary’s contract worthwhile for the consultancy running the authority. The National Union of Teachers, in its guidance on fixed-term and temporary contracts, says that they have long been misused as a way of dealing with school budget problems.

In Mary’s case, the school’s action is illegal if it can be proved that the contract has not been renewed specifically because she is pregnant. The problem, particularly for a teacher like Mary, who wants to get on in her career, and cannot face confrontation with her employer, is proving it. So uncertain can be the circumstances which necessitate the creation of an “acting up” post that its duration, terms and conditions lend themselves to manipulation.

“Sometimes you can be sure it is discrimination,” says Barry Fawcett, of the NUT, “but proving it can be another matter.”

So what are your rights when you are on a contract? The message both from the teaching unions and labour legislation is that teachers on fixed term or temporary contracts have, on paper, the same rights as their permanently employed colleagues. But teachers have to ensure that they have been clearly drafted to respect those rights.

For example, do you know the difference between “fixed term” and “temporary”? This is important, because a fixed-term contract has a far clearer end date. It may be used to cover a period of leave or inservice training where the finishing date is known, or - as in Mary’s case - to fill a post pending a specific appointment being made.

A temporary contract may be used for sickness or maternity cover, or for a short-term increase in workload with no precise finishing date. Budget trimming, back door staff cuts or unofficial probationary periods are not acceptable grounds for offering these contracts.

“The contract should state that you are in your acting position until, say, a head is appointed, or the school amalgamates - there should be a definite end,” says Magnus Gorham, of the National Association of Head Teachers. “We would have concerns if someone was appointed for one term then made redundant for no clear reason.”

The problem, often, is that being offered an acting-up post is hugely flattering. For many ambitious young teachers it is the first step up the ladder. It can seem churlish to step back and take a long, hard look at the post, and maybe even insist on certain guarantees or clauses being added to your contract.

But, Barry Fawcett says, this is precisely what you should do.

“Ensure the conditions of the appointment are clear,” he says. “There can be many different circumstances in which someone is asked to act up. It is important to establish those circumstances early on. As a matter of good management, there ought to be a clear timetable for the appointment and a clear understanding of what it entails.”

On pay, anyone acting up is entitled to a salary in the same range as the previous jobholder, but commonsense must apply. If you’re a relatively inexperienced head of year acting up as deputy head, you should not expect to be on the same salary point as a predecessor who has been in the job for years.

Of course things have been made more difficult by the dearth of headteachers. A headship may be advertised three or four times before it is filled. In this situation, the clearly specified finishing date favoured by the unions is simply not possible. An acting head may find their contract renewed again and again, while the search for a head goes on, for several years. Is there a case for extending to teachers the blanket right the European Union gives other contract workers to permanent posts once they have been doing the same job for two years?

“A blanket law which makes an acting head a head after two years would be enormously difficult,” says Magnus Gorham. “Each situation is different, and they have to be decided on casework. The difficulty in filling posts can mean that a school has to keep a deputy head on for as long as it takes - and that could be more than the two years stipulated by EU law,” he says.

Even if the acting head applies for the headship, says Mr Fawcett, there may be perfectly justifiable reasons, such as lack of experience, for turning him or her down.