A teacher was able to film himself abusing young girls because of a "lamentable failure" by school management, a review has found.
Nigel Leat, 51, was jailed indefinitely for abusing children at Hillside First School in Weston-super-Mare.
At his trial, which heard Leat abused five victims, some as young as six, the judge described him as a "paedophile of the most sickening order".
The serious case review was commissioned after Leat's 2010 arrest.
Leat, of Bloomfield Road in Bristol, admitted 36 sexual offences at Bristol Crown Court in May last year.
Thousands of images
The charges included one count of attempted rape, 22 of sexually assaulting a child under 13 and eight of sexual assault by penetration.
Police also found more than 30,000 indecent photographs on memory sticks.
The offences were committed between September 2006 and December 2010, when Leat was arrested.
A report carried out by Leat's school while he still worked there referred to at least 30 incidents of "inappropriate or unprofessional conduct" involving the teacher.
The serious case review, commissioned by North Somerset Safeguarding Children Board, said the incidents in the school management report ranged from indecent touching to inappropriate lesson content and over-familiarity with children.
"On a number of occasions colleagues advised the teacher of the inappropriateness of his behaviour and pointed to the risk that he could be accused of professional misconduct," the review said.
"However, only 11 of the 30 recorded incidents were reported formally within the school."
The incidents included Leat taking photographs of pupils on his mobile phone, and kissing and cuddling them.
A teacher also saw Leat in his underpants and a T-shirt while getting changed in his classroom.
Leat joined the school in September 1995 as a newly-qualified teacher.
The case review said a number of school staff had "a variety of concerns" about him within a year of his appointment and throughout the time that he worked there.
"Early on it was noticed that the teacher had favourite pupils within his class who were invariably girls, who were often given tasks within the class which were viewed as privileges as well as being given greater personal attention by the teacher," the review said.
"These pupils were allowed to be over-familiar with the teacher, who was known to speak and joke with his pupils in a manner which was inappropriately adult.
"This situation was described by staff to have been common knowledge amongst the school staff."
The review identified 20 pupils who were witnesses or victims of abuse by Leat.
It added: "Much of the behaviour exhibited by the teacher was typical of grooming activities pursued by adults intent on sexually abusing children.
"The failure of school managers to take action in response to the concerns raised was compounded by the failure of anyone in the school to recognise that the teacher's behaviour might have constituted grooming for sexual abuse.
"This raises questions about the impact of the safeguarding training that staff in the school had received."
Tony Oliver, independent chairman of the North Somerset Safeguarding Children Board, said: "Nigel Leat and Nigel Leat alone was responsible for the criminal behaviour at Hillside School.
"There is absolutely no suggestion that anyone else was involved."
He said the review report showed that the culture of safeguarding children at the school needed to be "much stronger".
The review's 32 recommendations include that the serious case review is read by head teachers, governors and safeguarding boards across the country.
The council's safeguarding children board met parents before the release of the report.
Sheila Smith, from North Somerset Council, said: "Clearly there is a range of views.
"Parents were made aware of the recommendations.
"For some parents they were satisfied that what we set out to do as a board back in December 2010 that we have done that.
"For some parents, I understand that they have concerns about can we ever guarantee that this is it now.
"For other parents they have actually chosen not to attend the meetings and have moved on in their lives."
Hillside First School was identified by Ofsted as academically successful.
Jean Humphrys, Ofsted's director for education, said no matter how rigorous its inspections, "Ofsted is not able to guarantee that inspectors will be able to uncover details of specific child protection cases".
He said: "This shocking case highlights to us all the need for constant vigilance in matters of child protection and I extend my sympathies to those affected.
"As the review acknowledges, Ofsted made considerable changes to the way it inspects safeguarding in schools in 2009 and provided additional training for inspectors."
In a statement, NSPCC Director of Child Protection Advice and Awareness, Peter Watt said: "This is a shocking case where young children were left at the mercy of a determined sex offender in their own school even though many people had concerns about Nigel Leat's behaviour.
"There were clear signs this man was a danger yet astonishingly the proper action was not taken, leaving children at risk when they should have been in a safe environment."
Leat was given an indeterminate sentence and told he must serve at least eight-and-a-half years before he can be considered for parole.
Last month the school's head teacher Chris Hood left his post, having been suspended since January 2011.