Online safety is a "high priority" for schools
School computer systems in the UK are failing to identify 10% of incoming harmful content, research suggests.
Data monitored from 30,000 students found content from items such as mobile phones and cameras which had not been picked up by filter systems.
Researchers from online safety firm E-Safe Education say children are still able to access inappropriate content.
The government's technology agency for schools, Becta, said systems were tackling this "real problem".
Students' data was monitored by E-Safe Education, checking how much inappropriate content was evading the schools' usual blocking and filter systems.
The firm's managing director, Colin McKeown, said: "A lot of schools have internet filtering, but they may just filter words and text.
"We are seeing a new generation of multimedia content and images downloaded from mobile phones, pen drives and CDs and DVDs which do not always have words attached."
"Schools can be really surprised by what they see."
He said internet filters would not alert schools to the attempt to download inappropriate content, and would potentially allow a child to access material which had no text clues to its content.
A spokeswoman from Becta, the government's technology agency for schools, said the possibility of downloading harmful content onto school machines was a "very real problem that schools needed to be aware of".
"The internet is a fantastic tool but it poses risks," she said.
"Schools have a responsibility to educate their pupils about the internet, but it can build and enhance pupils' learning and revision.
"This is an area which is constantly changing.
"The feedback we have is that schools place a very high priority on it."
She added that Becta provided a list of approved suppliers of computer safety systems for schools, and schools could be "confident and reassured" that these were strong and robust systems.
Guidance on computer and internet safety is also available for teachers, she said.