Appearing at St Albans Crown Court, Judge Andrew Bright QC said he will sentence Carl Langdell on Friday.Prosecutor Ann Evans said: "The tragedy of this case is that Katie Locke, like thousands of other young people, having agreed to a date with Carl Langdell accepted what he told her about himself.
He also admitted that he wanted to see her naked and have sex with her when she was dead.
Read more: Teenage girl with mental health problems died after Priory staff neglected her Katie had taught history and politics at Cardinal Pole School in Hackney, east London.
Before they met, Langdell lied to Katie when he said he had his own law firm.
He met her for the first time on the night she died.
Katie was unaware that Langdell, 26, spent time in a psychiatric hospital and was serving a suspended sentence for telling the nurse his sick fantasies.
Following his arrest he described himself to a psychiatrist as a "monster".
Langdell, of Cheshunt, Herts, admitted at an earlier hearing to murdering Katie Locke in the early hours of last Christmas Eve.
A sick and twisted fantasist violently assaulted and strangled a young teacher he met on the Plenty of Fish dating site before having sex with her dead body, a court has heard.
Carl Langdell met Katie Locke on the dating app Plenty of Fish.
When they met up, he took her back to the four star Theobalds Park Hotel in Hertfordshire where he violently attacked and killed her.
St Albans Crown Court heard how he dumped her body in a bush near a skip in the 55-acres grounds after strangling her to death.