Zahid Akram, 37, a physics teacher at a school in Dorset county, would persuade his victims to send him pictures in their underwear and then threaten to forward them on to their family if they did not send more, the Daily Mail reported Tuesday.
Detectives believe Akram, a father of two, may have contacted hundreds of youngsters on Badoo, a social network, while posing as a schoolboy.
Akram was warned by a judge at Exeter Crown Court that he faces a long spell behind bars after he admitted to four charges of blackmail and one of causing or inciting a child aged 13-17 to prostitution or pornography.
Police believe there are victims in Scotland, Wales, Northern Ireland and in England.
Officers said his demands were about "empowerment rather than sexual gratification". Akram had a dozen different profiles on Badoo through which he used to coerce his victims into handing over pictures.
Judge Francis Gilbert QC refused bail to Akram, of Redditch, Worcestershire, and 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."
In one case, the judge said, Akram told a schoolgirl he knew the Christian names of her parents and would show them the pictures unless she sent more sexually explicit images.
The blackmail and incitement started in October 2009 and continued until January 2012.
The judge warned him that he faces a prison sentence measured in "years not months".
Akram will be sentenced at Exeter Crown Court Nov 14.
--IANS (Posted on 15-10-2013)