Phillip Barrett, 32, former director of sport at Eggbuckland College in Plymouth, Devon, admitted two sex offence charges.
Plymouth Crown Court heard Barrett had started a conversation on the Kik social media site.
He was given a three year community order at Plymouth Crown Court.
He was also ordered to take a rehabilitation course for sex offenders.
Barrett, of Ivybridge, admitted charges of attempting to get a 14-year-old girl to engage in sexual activity and inciting her to watch sexual activity.
Barrett pretended to be 23 years old when he started the conversation last November, the court heard.
The conversations escalated in December - leading to a seven-minute Skype call in which he tried to persuade the "girl" to watch his sexual act.
Barrett immediately confessed to police after he was arrested in January, the court heard.
Barrett was under stress at work and had relationship problems, the court was told.
But Judge Ian Laurie said he was "at a loss to understand" how Barrett, who had no criminal record, could have committed the crimes.
He told Barrett he could have been jailed, but said a "welter" of mitigation led him to believe the teacher was "in essence a good man".
"I'm reasonably confident this court will not see you again," the judge told Barrett.
Det Con Darren White of Devon and Cornwall Police's child exploitation unit said it was "up to the public" to decide if the sentence was fair.
"The internet gives people a sense of anonymity and people need to be aware that online activity can be traced," he said.
Barrett, who has been sacked from his job at the college, was also ordered to pay Â£700 prosecution costs.
A PE teacher tried to persuade a 14-year-old girl to watch him in a sexual act via Skype - but the girl was an undercover police officer.