Education ministers appear to have broken their own rules by allowing teachers who had committed offences which should have led to automatic, permanent bans to work in schools.
The irregularities are among a range of problems which undermine the system for vetting staff.
Department for Education and Skills guidance on child protection states there circumstances in which the Education Secretary "must issue a direction barring someone automatically".
These include teachers who have committed one of 42 offences since November 1995, such as indecent assault on a child under 16. Two of the offences - murdering a child or having indecent pictures of a child - only lead to automatic bans if they occurred after June 2003.
Automatic bans also only apply to those who are convicted in court. Paul Reeve, the Norfolk PE teacher cautioned by police for accessing child pornography, escaped inclusion on List 99 for this reason.
The date limits have allowed other school staff to avoid permanent bans.
William Gibson was convicted of indecent assault on a 15-year-old in 1980, but was not on List 99 because it occurred before 1995.
However, no such loopholes explain why Keith Hudson - and possibly several others - were allowed to teach.
Hudson was convicted in 1998 on five charges of importing indecent and obscene magazines showing young boys.
He was placed on List 99, but was later allowed to work in a girls' school by the then education secretary Estelle Morris.
The DfES said that teachers who commit automatic-ban offences - which include the importing child pornography - cannot appeal against List 99.
The General Teaching Council for England has suggested it could help the Government by taking over judging whether sex offenders should work in schools.
However, confusion exists over the disciplinary cases which the GTC already examines and teachers' unions fear it is too lenient.
The Education Secretary is supposed to examine cases relating to "the safety and welfare of the child". Yet several cases directly involving child welfare have been judged by the GTC. These include John Davies, former head of Swinford Manor in Kent, who was accused in October 2005 of kicking a pupil and using excessive headlocks on students.
The Education Secretary is also supposed to consider barring teachers who have been jailed for more than 12 months or have committed sexual offences, violent crimes, or behaviour "which indicates a risk to others".
But teachers who have committed all these crimes have been referred to the GTC, including one jailed for killing his wife. The most recent figures show that fewer than one in five teachers whose cases were heard by the GTC were banned for life.
Staff who have been allowed to return to teaching include Christopher Ogbe of Bexleyheath school in Kent, who was jailed for three years for indecently assaulting a colleague on a school trip. The GTC tribunal gave him a 12-month suspension in 2003.
A further loophole is that independent schools do not have to employ GTC-registered teachers or check if the council has barred them from teaching.
The launch of the Criminal Records Bureau in 2002 was supposed to improve child protection by drawing together information held by police forces and government bodies.
Initial delays meant that hundreds of schools had to employ teachers still waiting for clearance.
The service has since improved and last year more than 90 per cent of full checks were carried out in four weeks.
But Pauline Latham, former director of Safer Recruitment, said that frustration with the CRB was part of the decision why she closed her vetting agency last year.
Mrs Latham said she never received responses to more than 200 applications she had posted in 2002, which were lost in the system.
"It's bizarre," she said. "I just have to assume that the teachers put in applications for checks again with other agencies later or moved on to other jobs."
A Home Office spokeswoman admitted that "some police forces had been slower than others" in providing information and that it was working with them.
Lost in lists
Adults can be listed on an array of databases if they have committed acts that might make them unsuitable to work with children.
These include local police databases; the sex offenders' register; List 99, which is overseen by the DfES; List 98s, overseen by local authorities; the Protection of Children Act (POCA) and Protection of Vulnerable Adults (POVA) lists.
The information from all these lists should be collated when a CRB check occurs. But the DfES and the Department of Health admitted in a briefing paper last month that "current barring list inconsistencies" existed.
They also noted inconsistencies in the way police forces disclosed information and local employers made decisions.
Frequently asked questions
How can schools check whether someone is considered unsuitable to work with children?
A job applicant's Criminal Records Bureau disclosure form should list police records of convictions, cautions, and reprimands, and whether they are on List 99, the Protection of Children Act list and Protection of Vulnerable Adults list. It does not include information from List 98s held by local authorities. Heads are advised to check references thoroughly and use interviews to spot potential child abusers.
Who should vet supply agency teachers?
Supply agencies are responsible for ensuring that checks have been carried out on staff they provide, as they are their employers. But the DfES says it also expects schools to contact supply agencies to make sure the teachers they hire have been vetted.
Should teachers who have been arrested accept police cautions?
The National Union of Teachers is urging members to refuse to accept cautions if they are innocent as these could be treated as admissions of guilt and their names could be added to the sex offenders' register or other lists.