Pupils have been discovered illicitly accessing prohibited information, such as teachers' addresses and personal details. Other confidential information is also at risk.
"At one place they had put the internal exam papers on the network, so the students got in to see those as well," said Andrew Hope of Huddersfield University, who examined computer use at eight institutions to assess the risks posed by the Internet.
His work suggests schools should focus on these dangers rather than their current main Internet worries: on-line paedophiles, race-hate sites and pornography sites. The risk from paedophiles was "statistically remote", he concluded, while children were "not likely to suffer psychologically as a consequence of briefly viewing pornography".
Mr Hope said teachers should be concerned at the ease with which pupils can hack into school networks. Most schools keep administrative files separate from the main network, but those that do not, he warned, were at risk frommischief-makers.
The main focus of the paper was on how the attitudes of teachers to the Internet varied depending on which age group they taught. While primary teachers talked solely about pupils being "at risk" on the Internet, secondary teachers saw their students potentially harming the school's reputation by, for example, making obscene entries on websites.
"Copies of The influence of students' ages upon staff perceptions of school internet hazards" by Andrew Hope will be available from the author later this year