According to the Police College of Finland, online offences range from obscene language to offers to pay for sex. Other youth are the most frequent perpetrators.
Parents rarely monitor their children's use of the internet, according to Noora Ellonen of the Police College.
"Girls in the ninth grade often reported the use of obscene language online. They were often asked for sexy pictures of themselves. About five percent of 15-year-old girls said they were offered payment for sex," Ellonen said.
About ten percent of sixth grade students said they had been victims of virtual harassment. Girls are more frequently targeted than boys. About 20 percent of 12-year-old and 15-year-old boys said they had been victims of online sexual harassment.
Officials say online sexual harassment offences rarely end up in court.
Some 13,500 pupils in grades six and nine were interviewed in the survey.