Ministers are urging schools to involve police in the most serious cases of cyberbullying of pupils and staff in a new crackdown on the "unaccceptable" and "insidious" use of new technology.

They say some instances might break criminal or civil law protecting people from threatening communications and could lead to Asbos. Police intervention, along with that of mobile phone and web companies, might be needed to track the digital footprint of perpetrators.

The threat of legal sanctions on top of school disciplinary measures, including the confiscation of mobile phones and temporary bans or restricted access to the internet, is recommended in far tougher guidance to combat the increasing incidence of this form of bullying.

Research for the new campaign found one in three 12-15-year-olds reported having been cyberbullied, a higher proportion than other surveys. Teacher unions, too, say members are being subjected to demeaning and offensive attacks through pupils' videos and comments on websites.

The drive includes a new emphasis on curbing "bystander bullying" by young people who pass on images or comments made by the original bully without thinking. Sanctions should also be devised to deter these "accessories", according to the government, which is using on-line pop-ups to warn any young people tempted into this behaviour: "Bullying causes depression, self-harm and even suicide. Laugh at it and you are part of it."

The campaign, drawn up with the help of industry, gives practical tips including not responding to malicious texts or emails, saving evidence, reporting incidents, keeping passwords safe and not revealing details such as mobile phone numbers over the internet. Parents should also be fully involved.

Ed Balls, the children secretary, said: "Schools need to get to grips with newer forms of bullying and the different impact these have on the victim ... bystanders can inadvertently become perpetrators. Simply by passing on videos or images, they are playing a part in bullying. It is important pupils are aware that their actions have severe consequences and participating in such activity will not be tolerated."

Unions welcomed the measures, although Steve Sinnott, general secretary of the NUT, said: "As good as the guidance is, parents, teachers and young people will still need the assurance that when a complaint is made to a provider offensive material is removed."

Ministers should monitor the effect of the guidance closely to see if further changes in the law were needed.

Ministers will today also challenge homophobic bullying, saying it must become as unacceptable as racist remarks. They were shocked by a report by the charity Stonewall which said homophobic remarks were still common in everyday conversation.