David Warbrick 51, pleaded guilty to offences against the two victims, who are both from Derbyshire, at Derby Crown Court,
Derbyshire police say David Warbrick set up a false account and began chatting to the youngsters.
He then asked them to go on to a webcam and take their tops off, a spokesman said.
But the mother of one of the victims discovered the online web chat conversation and the police were called.
They arrested and charged Warbrick with two counts of causing the underage girls to engage in online sexual activity.
A police spokesman said: “The offences took place in August 2013.
“Mr Warbrick created a false Facebook identity and contact the two girls, who were 11 at the time and were from Ilkeston.
“He began chatting to them on Facebook at first and then invited them to a web chat.
“He told the girls his camera was not working so they were unable to see him but he could see them.
“He then asked them to take their tops off for the camera.”
Warbrick, of St Giles Road, Lightcliffe, will be sentenced next month.
He was head teacher at All Saints Junior and Infants School in Halifax until 2010.
Calderdale Council’s Director of Children and Young People’s Services, Stuart Smith, said: “Mr David Warbrick was employed as head teacher at All Saints Junior and Infant School from 2004-2010. The charges relating to Mr Warbrick have no connection with any current or former pupils at this school and are not linked to Calderdale in any way.”
He was suspended as head of Gomersal Primary School, Cleckheaton, after his arrest in 2015.
A spokesman for Kirklees Council said: “We can confirm that David Warbrick was suspended from his post as soon as these allegations came to light some time ago.
“He did not return to school prior to leaving his post permanently in 2015.
“It is important to stress that the offences have no connection whatsoever to David Warbrick’s former school.”