few weeks before Easter, MPs questioned senior figures from the Office for Standards in Education about disaffected pupils. What, they wanted to know, was a pupil-referral unit for?
It was a good question, David Taylor, the Ofsted's director of inspection, admitted, and one which had been giving his organisation cause for concern.
These units were dealing with some of the most difficult pupils, and generally they were ill-equipped to cope with them, he said.
"Few units have staffing, accommodation or resources which enable them to meet fully the needs specified in pupils' statements," he said.
"If you start with what is a PRU really aiming to do, it is not a repository for special educational needs students who you cannot cope with."
The question goes to the heart of the ongoing debate about how local authorities should cater for children with behavioural difficulties. David Bell, chief schools inspector, told the same education and skills select committee hearing that this was "the major flashpoint between schools and their local authorities".
This week Ofsted reported that teaching in one in five pupil referral units was unsatisfactory while management in one in four was not up to scratch.
So what are pupil referral units supposed to be doing? And are they doing their job or are they, as David Taylor seemed to imply, simply becoming dumping grounds for difficult pupils - overstretched, underfunded and often inadequate?
According to legal guidance on PRUs published by the Department for Education and Skills in November 1999, these units should not be a final destination for an excluded pupil. Every boy or girl entering a PRU should have a plan for his or her reintegration into a mainstream or special school, it says.
But PRU managers have told The TES this aim is not being met. One head of a unit in the London area said many of her pupils had been with her for four or five years. She also said her LEA was not helping to ease her situation.
"In my area you can't reintegrate children because there are no school places to reintegrate them to," she said.
"Some children from the primary PRU go through into the secondary PRU without ever having a place at a mainstream school. They get excluded from their primary schools and don't see a mainstream school again.
"My authority's policy lacks structure, strategy or vision. Dealing with them is like walking through treacle."
The DfES guidance adds that, while significant numbers of pupils entering PRUs are bound to have statements of special needs, these units are not the appropriate places for statemented pupils' long-term education. PRUs should never be named on pupils' statements as their places of education, it says.
Yet more than one PRU manager among those interviewed by The TES said they had fought off moves to name their units on pupils' statements.
Government statistics show the proportion of PRU pupils in England who are statemented is much higher than the national average - 18.5 per cent in 2002, compared with 3 per cent across all schools. This is not surprising, given the PRU's role as a short-term destination for excluded pupils.
A glance at the figures over time is encouraging. In 1995, the first year for which figures became available after the PRUs were established in 1993, 26.3 per cent of pupils in PRUs had statements.
However, a closer look at these statistics reveals a more disturbing picture. Over a period in which a key aim of the Government's special needs policy has been to integrate SEN pupils into mainstream schools, the proportion of statemented pupils who find themselves in PRUs has actually gone up, from 0.6 per cent in 1995 to 0.7 per cent in 2002.
This might seem contradictory, given that the proportion of PRU pupils with statements is actually falling. There is a simple explanation. In the past seven years, the total number of pupils in referral units has doubled, from 5,000 to almost 10,000. In the same period, the number of statemented pupils in PRUs has risen by 40 per cent, from 1,300 to 1,800. The total number of statemented pupils in England has risen more slowly, by 18 per cent. So statemented pupils are becoming ever-more likely to find themselves in referral units.
This, as David Taylor acknowledged, has left these units struggling to cope. While their pupil numbers have rocketed, teacher recruitment has failed to keep pace, and their pupil-teacher ratios have risen from 3.5 to one in 1995, to 4.4 to one in 2002.
The latest Ofsted report is one of several worrying indicators of the standard of education provided by PRUs.
PRU managers say they do the best possible job in very difficult circumstances, and believe it is inevitable that some of their units will be judged to be failing. Their difficulties are exacerbated by poor accommodation.
These problems have become more acute since last September, say PRU managers, because of a new rule that obliges local authorities to provide full-time education - 25 hours per week - for all excluded pupils.
PRU managers who spoke to The TES said they had been unable to fulfil this obligation. Some said they had had to admit defeat on the issue; others said they were giving their pupils work to do at home. Some added that for many pupils the measure was counterproductive as they were unable to handle five hours a day of schooling.
"The staff aren't there to actually teach these kids. It's physically impossible," one PRU headteacher said. "A third of my children are on part-time timetables because my staff don't have the skills and support necessary to teach them for the full 25 hours. But to be honest, from the kids' perspective, they simply couldn't manage 25 hours."
Many PRU heads argue it is the Government's rules, and not their practice, which is wrong.
Ian Martin, deputy head of the Mendip Centre, a PRU in Glastonbury which was named as outstanding recently by Ofsted, says he knows of a PRU in special measures where one issue was the lack of full-time education. He says his centre's policy is to do what is appropriate for each child.
"There are some statemented kids for whom 25 hours would be a disaster.
They are not emotionally equipped for that kind of intensity. Some kids do not want to be in mainstream schooling and it is not in their interests.
"One kid, in from the age of eight, said, 'I'm not going back to mainstream, I like it here'. He's left us now, has got a job and is happy.
"What would have been the point of manhandling him back to the mainstream?"