Richard Swinnerton said he had watched child sexual abuse on the dark web for two years before he was arrested by National Crime Agency officers at his home on May 15 this year.
The 30-year-old, of Coulby Newham, Middlesbrough, told investigators the footage including adults raping children.
Swinnerton – who has resigned from his job at St Clare’s Catholic Primary School in Middlesbrough – said he used TOR software (The Onion Router) to make himself anonymous on the dark web.
Officers have found no evidence to suggest that any of the images were made at the school and there is no allegation of physical abuse.
Swinnerton appeared at Teeside Magistrates’ Court and admitted three counts of possession of illegal images of children relating to category A (the most severe), category B and category C.
When officers arrested him they seized his laptop which contained 87 Category A videos of child abuse.
He will be sentenced at Teeside Crown Court on a date to be confirmed.
NCA operations manager Julie Booker said: “Swinnerton had gone to considerable lengths using TOR to commit his offences and thought he was anonymous on the dark web.
“Offenders who view child sexual abuse are contributing to ever more children being abused and condemning them to years of extreme trauma and suffering.
“Every child in an abuse image is revictimised when the photograph is viewed or shared.
“Protecting children from sexual abuse and stopping paedophiles is a top priority for the NCA.”
A St Clare’s school spokesperson said: “This has been an upsetting matter for parents, carers, staff and the wider St Clare’s community.
“We are grateful for the support we have received from them and we continue to assure them that the wellbeing and safety of children of St Clare’s remains our priority at all times.
“It is also important to reiterate that this former employee’s offences were not in connection with his employment at the school.”
Speaking after the hearing, an NSPCC spokesperson said: “Swinnerton went to horrifying lengths to access this awful material and his actions have fuelled demand for an online trade that must be stopped.
“This is yet another clear example of why the UK Government must urgently push forward with an Online Harms Bill that would force internet companies to take further action to prevent such depraved content from being uploaded and shared online – which we have been demanding through our Wild West Web campaign.”