Pupils are quoting fake news as fact in lessons and written work, it has been suggested.
More than a third of teachers say that students have cited false information they have found online, according to a poll by the NASUWT teaching union.
Union general secretary Chris Keates said the finding was "worrying" and shows the power that internet firms have in shaping public opinion, especially among young people.
The finding, part of an NASUWT poll on social media and technology, comes just weeks after a leading international education expert said that children should be taught in schools how to recognise fake news.
In one case, a union member said that "some students did not attend school and hysteria ensued because they thought there were killer clowns roaming the streets with weapons".
Another said that pupils "often mistake spoof news sites for real news".
In total, 34% of the union members surveyed said that in the past year they have seen pupils citing clearly fake news or false information from the internet as fact in their work or classroom discussions.
"It is worrying that over a third of teachers had experienced pupils citing fake news or inaccurate information they had found online as fact in their work or during classroom discussions," Ms Keates said.
"This demonstrates the great power that companies such as Facebook and Google now have in shaping public opinion, particularly among young people who have never known a world without internet and who are less equipped to analyse the information they see presented to them online and assess its plausibility.
"It is important for children and young people to be made aware that not everything they see and read online is real."
She said that teachers are trying to help educate pupils when they cite false information, but added that as with other forms of technology misuse, it is important for online providers to "take responsibility for the material hosted on their platforms and to take steps to tackle those who seek to misuse these sites".
Last month, Andreas Schleicher, the Organisation for Economic Co-operation and Development's (OECD) director of education and skills, said that in the modern digital age, schools should teach pupils how to think critically and analyse what they read on social media and news sites.
"In the past, when you needed information, you went to an encyclopaedia, you looked it up, and you could trust that information to be true," Mr Schleicher said.
"Distinguishing what is true from what is not true is a critical skill today," he added.
"Exposing fake news, even being aware that there is something like fake news, that there is something that is written that is not necessarily true, that you have to question, think critically, that is very important.
"This is something that we believe schools can do something about."