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.
Education Workforce Council figures, external 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, external said it was not a common occurrence in schools.
In total, 62 teachers were punished at disciplinary hearings in two years. The actions of seven others did not amount to unacceptable professional misconduct or serious incompetence
The 13 teachers were removed from the register for:
Nov 2013 - Having inappropriate and sexually explicit conversations with pupils in a Year 12 drama class and with one pupil on Facebook
Nov 2013 - Having a sexual relationship with a male former pupil
Nov 2013 - Engaging in sexual activity with teenage boys from the school, giving them drugs and alcohol and creating inappropriate photos
Dec 2013 - Using sexual language, which caused distress to a child, and having inappropriate conversations with pupils
April 2014 - Developing inappropriate relationships, exchanging emails and having inappropriate physical contact with female pupils
May 2014 - Making inappropriate references to material with sexual or adult content and showing inappropriate internet material whilst teaching
May 2014 - Having inappropriate conversations with female pupils
June 2014 - Having a sexual relationship with a 15-year-old boy
June 2014 - Inappropriately hugging and kissing pupils and encouraging inappropriate social contact with students outside school
Sept 2014 - Having inappropriate contact with former pupils
Sept 2014 - Sending inappropriate text messages to a sixth form pupil
Nov 2014 - Exchanging phone messages of a sexually explicit nature with a secondary school pupil
July 2015 - Sending sexually explicit messages to a pupil via text, Facebook and Skype
Some of the cases involved inappropriate conversations sent via text message
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, external, 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, external 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.
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