But NASUWT General Secretary Chris Keates said the evidence collected on its own website demonstrated that teachers were still being targeted.
One teacher gave the example of pupils who had created a webpage on which abuse, intimidation and even death threats were made about school staff.
Another posted information about a "We Hate Mrs XXX" website created by a pupil after the teacher in question reported them for an incident.
One teacher claimed she had received 68 malicious mobile phone calls over a three-week period in which she was threatened with rape and murder.
Ms Keates said: "The pupils who do this are afforded anonymity. The teachers are named, exposed to ridicule and subjected to false and malicious allegations.
"Teachers' self-esteem and sometimes their health is seriously affected."
She also warned that teachers' chance of getting new jobs could be harmed by the web abuse.
"Publicly available, derogatory remarks about a teacher's practice can damage career progression if those advertising jobs decide to trawl these sites."
Confiscation powers
A Department for Education and Skills spokesman said: "We are clear that respect for teachers is a must in all schools.
"Our tough new behaviour powers give teachers a clear legal right to confiscate items, including mobile phones, from pupils who misuse them.
"We are also currently updating our cyber bullying guidance to schools to ensure they know how to tackle this new and emerging issue.
"We are also working with a range of websites, phone companies and internet service providers to look at what they can do to reduce the problem of cyber bullying in schools."
