By Mike Baker BBC education correspondent

I cannot remember such a chaotic start to the school year.

On the first day of term, I was at a London secondary school.

Two year groups had been told to stay at home.

Why? Because seven of the school's 36 teachers had still not been fully vetted by the Criminal Records Bureau.

No joke

I met one of the teachers. She was having to work in isolation in the science staffroom.

Outside one wag had stuck up a notice warning: "Beware Dangerous Teachers".

But it wasn't really very funny.

By the rules, she was not allowed to be in a classroom, even under supervision.

Oddly enough, my television crew and I (three men in a girls' school) were allowed greater freedom.

No discretion

She was an experienced teacher who, until July, had been working at an independent school.

Although there had been no break in her career she needed full vetting for her new job because her previous school had not gone through the full procedures.

The school's head teacher was quite willing to use her discretion, relying on references and other information, to permit her new teachers to start work.

But she was not allowed to. The girls stayed at home, the teachers tried to keep busy with lesson planning.

Avoidable

Was all the disruption that schools throughout England and Wales faced this week really necessary? And who is to blame?

The answer to the first question is easy: no, the disruption was avoidable.

The answer to the second requires the Judgement of Solomon.

The reason it was avoidable was that almost as soon as the Criminal Records Bureau took over the new system of vetting back in March it became clear that the scale of the task had been vastly under-estimated.

There was also the unanticipated problem that most applicants preferred to apply in writing rather than by telephone. This meant errors took much longer to rectify.

Whatever the causes, the backlog was substantial by May.

Rules changed

That was when the Department for Education and Skills wrote to the Home Office (which is responsible for the Criminal Records Bureau) to express its concern.

Although it lacked clarity, this compromise position was sensible

The Education Secretary, Estelle Morris, saw her former boss, David Blunkett, now the Home Secretary, to discuss it.

As it became clear the backlog would not be cleared quickly, the government - in guidance which had the clarity of mud - left head teachers and supply agencies with the impression that, while these problems were sorted out, teachers could begin in the classroom without the final police records check.

Although it lacked clarity, this compromise position was sensible and, if ministers had held their nerve, it would have avoided the disruption that marked the start of term.

And then ...

The compromise position still required all the checks that had been in place before March.

Teachers still had to be checked against the government's List 99, which contains names of those considered unsuitable to work with children.

Schools also had to take up references and testimonials.

They also had to check with local police records, although teachers could begin work before these check (which could take months) were completed.

But then came the terrible events in Soham.

The media and the public went into a panic reaction about child safety. The government - ever watchful of the public - tightened up its guidance.

Rules changed again

Now only teachers who had been through the full, enhanced checks could teach.

The final, red warning signs were there

In short, a List 99 check, references and checks with previous schools or training colleges were no longer enough. All police files had to be checked by the bureau.

Yet this new, tougher, requirement was brought in just days ahead of the new school term.

In fairness to education ministers, they say the bureau assured them the priority list of teacher applications would be processed by the start of term.

The final, red warning signs were there when schools in areas where term started earlier than elsewhere, like Leicestershire, had to send pupils home.

No word

Despite the bureau's 1,100 staff working through the night and the weekend, by the start of the week when most schools were due to return there were still about 10,000 applications awaiting processing.

There was still time to avoid full-scale disruption. Ministers could have relaxed the new rules.

After all, in doing so, they would only be going back to the situation they had agreed in May.

The alternative was a chaotic start to the school year and, quite probably, the risk of placing many children of working parents in even greater danger because many would be unexpectedly at home without supervision.

Monday passed, Tuesday passed and most of Wednesday too. Not a squeak from Whitehall.

Announcement

It was clear that hundreds of schools were unable to provide for all their pupils.

Head teachers seethed with frustration.

I don't doubt for a moment that ministers are genuinely concerned for child safety

Eventually on Wednesday evening, with the backlog of priority applications still at around 7,000, ministers acted.

Teachers' representatives were summoned hastily to the Department for Education.

Soon after 1800 BST the announcement was made to the news media: the post-Soham heightened restrictions were being relaxed again.

It was, of course, too late for most schools to contact pupils in time for Thursday morning. So many pupils missed a further day of school.

Perhaps, though, things could be back to normal by Friday?

Finally

However, as the hours of Thursday's school day ticked past, head teachers and local education authorities grew nervous.

They had heard from the media that the restrictions were being relaxed, and that schools could use their own discretion in allowing teachers to teach, but they had received no official communication.

The end of school bells rang on Thursday afternoon and still no word came.

The National Association of Head Teachers said the situation was still "on hold": its members would not be safe from legal challenge if they started using their discretion without written confirmation from above that this was being allowed.

Finally, in the early evening, the official advice was issued by e-mail to town halls. By Friday they could start planning a return to normality.

So, the disruption could have been avoided if the guidance had been changed sooner. Why wasn't it?

This is where it gets difficult.

So what changed?

I don't doubt for a moment that ministers are genuinely concerned for child safety.

It will take a full Parliamentary inquiry to determine the underlying blame

They had to make a tough decision on the balance between requiring full checks and the risks of leaving children out of school.

If the Criminal Records Bureau had really convinced them the backlog would be cleared by the start of term, then it was perhaps reasonable for them to want the best of both worlds: greater security and no disruption.

But there were good reasons to doubt that the bureau could meet its deadline.

It had been struggling for months. If, in May, ministers were convinced of the need to relax the rules, what happened in August to change their minds?

Well, despite denials by ministers that Soham played any part in this, we know that events in Cambridgeshire raised public concerns over child safety. It is hard to see any other cause of the ministerial change of mind.

Inquiry

Reacting suddenly to short-term swings in public mood has proved dangerous for governments before. Think of the troublesome Dangerous Dogs Act, for example.

The government has vacillated.

It relaxed the rules in May, tightened them again in August, then eased them again in September.

Not all those decisions can have been right.

It will take a full Parliamentary inquiry to determine the underlying blame for the vetting backlog: it could be the Home Office which is responsible for the Bureau, the private company that helps run it, the police for delays in response times, or employers and applicants for submitting papers with errors in them.

At present, its impossible to say.

But it was the changes in guidance, and the false assurances that the start of term deadline would be met, which unnecessarily triggered the chaos at the start of term.

Mike Baker and the education team welcome your comments at educationnews@bbc.co.uk although cannot always answer individual e-mails.