Education Workforce Council figures showed 13 of 34 staff were barred for such conduct from August 2013 to 2015.
Two more were suspended, while three others were banned for other misconduct involving sexual content.
The National Union of Teachers said it was not a common occurrence in schools.
David Evans, secretary of National Union of Teachers Cymru, said: "We have to be mindful when reviewing these figures that this is a very small number of teachers across the whole profession over a period of a few years. This is by no means a common occurrence in our schools."
He added the union often dealt with malicious and false allegations against teachers, which remained a concern, but said it was right allegations were investigated and any teachers found abusing their positions should face the full consequences.
Donald Findlater, of child protection charity the Lucy Faithfull Foundation, said the changing online environment and social networking in recent years had introduced "massive challenges and risks".
He said some staff got into trouble out of naivety, rather than malice, and schools must ensure adequate training is given so teachers know the boundaries.
A spokeswoman for child abuse charity NSPCC Cymru said: "Schools need to be rigorous in their recruitment and checking processes and provide regular training about appropriate behaviour, making it absolutely clear that crossing the line will never be tolerated."
During the same period, other teachers were disciplined after being convicted of driving, alcohol, drugs, harassment and fraud offences.
Others were punished for assaulting pupils, helping children cheat during tests, failing to achieve required standards, not following health and safety procedures, and failing to investigate bullying.

Sofia Ashraf, 29, posted a Facebook live video of her singing a song, My Vote Is Not For You, in front of the new chief minister's residence.
Sasikala Natarajan, a close aide to the late chief minister J Jayalalitha, was appointed to the post on Sunday.
The decision has fuelled anger as Ms Natarajan has never held public office.
She was made general secretary of the AIADMK party after Jayalalitha's death in December.
The recording of Ms Ashhraf's live video has been viewed 45,000 times and shared almost 1,000 times since it was uploaded to Facebook 18 hours ago.
Tamil Nadu to get second woman leader
The Indian rapper who took on a multinational
"The youth are getting increasingly agitated, and this appointment of a woman who has never stood for election being made chief minister was the straw that broke the camel's back," Ms Ashraf told the BBC.
"It has made us lose faith in democracy."
The AIADMK had earlier explained its choice of chief minister.
"Like Amma [mother], we wanted a strong woman leader to serve the people," party spokesperson CR Saraswathi told the AFP news agency, referring to Jayalalitha by her popular nickname.
Ms Ashraf says she was performing at a venue two streets away from Ms Natarajan's residence in Chennai (Madras) when her band decided to change the lyrics  to one of her songs.
She and others marched towards the house and transmitted it live on Facebook.
Police tried to stop the recording, but Ms Ashraf told the BBC that those accompanying her argued that she was doing nothing illegal.
"In Tamil Nadu there is a culture of bullying. There is no space for dissent. If you protest on the streets you are arrested, and if you protest on social media you are charged with defamation. This protest was my way of reclaiming both those spaces," she said.
"Party workers are asking us to give Sasikala a chance, but let her stand for election first."
Ms Natarajan's appointment caused a lot of chatter on Indian social media, both in Tamil Nadu and outside, with many expressing opposition to the decision.
Nearly 40% of teachers banned from the profession in Wales in the last two years have been struck off for having "inappropriate" relationships or conversations with pupils.
[NEXT_CONCEPT]
A young Indian rapper has taken on what she calls the "undemocratic" appointment of the new chief minister of southern Tamil Nadu state.