A head teacher destroyed a note alerting him to a teacher having sex with a student, a child abuse inquiry heard.
Headlands School in Bridlington previously had a "culture" of inappropriate behaviour and attitudes by staff, the inquiry was told.
Five staff members were convicted of sexual offences against children between 2006 and 2010.
During the hearing, Anthony Halford apologised to anyone who was "harmed".
The Independent Inquiry into Child Sexual Abuse (IICSA) heard the anonymous note was sent to him in 2000 about teacher Ian Blott having sex with a pupil in a cupboard.
Blott was jailed in 2006 after admitting four charges of sexual activity with a child under 18.
"I take responsibility for it. We destroyed it because it was monstrous really," said Mr Halford, who was head between 1991 and 2004.
He said he did not consider the letter as a child protection issue because he "thought it was a libellous note", adding: "I thought it was just lies."
The public inquiry heard there were a lack of procedures for reporting allegations of abuse.
Mr Halford and fellow staff were "not aware" of guidance on child protection and did not take safeguarding training, despite information being sent to the school by the council, and neither did he report "any concerns" of inappropriate behaviour to the local authority, the inquiry was told.
However, there was a "written code of conduct" for staff on how to behave with children.
Mr Halford told the panel the policies were overlooked because he was "more concerned about false allegations against staff".
But he said he was "appalled that these things happened" and described it as "an aberration".
He said he had been manipulated by perpetrators including Blott and another teacher Steven Edwards, who was jailed in 2007 after having sex with three pupils.
"I was so determined to protect Edwards from what I saw was harassment that I didn't properly listen... and I deeply apologise to the parents because I feel like I let them down. I failed them."
The hearing is part of an investigation into incidents of sexual abuse at schools which will also include Clifton College in Bristol and Hillside First School in Somerset.