A married physics teacher blackmailed schoolgirls into sending him sexually explicit photos of themselves in their underwear.
Detectives believe father-of-two Zahid Akram, 37, may have contacted hundreds of youngsters on the social networking site Badoo while posing as a schoolboy.
He would persuade his victims to send him pictures and then threaten to forward them on to their family if they did not send more.
At Exeter Crown Court he admitted four charges of blackmail and one of causing or inciting a child aged 13-17 to prostitution or pornography.
Judge Francis Gilbert QC refused bail to Akram, a science teacher at Woodroffe School in Lyme Regis, Dorset.
He said: "He knew perfectly well what he was doing. What he was doing was extremely nasty.
"He persuaded a number of young girls to take off their clothes and show themselves wearing their underwear, bra and knickers."
The judge said in one case he told a schoolgirl he knew the first names of her parents and would show them the pictures unless she sent more sexualised images.
"It put appalling pressure on her. It is a vile offence," the judge said.
"He is an intelligent man, a teacher, he must have known what he was doing. It was appalling behaviour."
Detective Constable Steve Harris, of Devon and Cornwall Police who investigated the case, said Akram, who comes from Redditch, Worcs., blackmailed vulnerable girls and young women aged between 13 and 30.
"I think there are hundreds of victims he has contacted. They are aged between 13 and 30 and they live all other the UK, in Scotland, Wales, Northern Ireland and from Bristol upwards," he said.
"If they refused he threatened to show the pictures to friends and family or post them online. In some cases he demanded money from them - £20 or £40 to use to top up his phone. It wasn't a vast amount of money but these were vulnerable victims.
"He was blackmailing these women. His demands were about empowerment rather than sexual gratification. He had very little control over his life at work or at home.
"He also had £40,000 debts and he was not very good with money. He needed some way of taking control and this is what he ended up doing."
Akram will be sentenced at Exeter Crown Court on November 14. The judge warned him that he faces a prison sentence measured in 'years not months'.