Cyber criminals have stolen more than 320,000 files from schools in Bristol and released them on the dark web.
The data was taken from a server used by the schools to store sensitive information about pupils, including educational psychology reports.
Bristol City Council said personal data belonging to 39 schools had been stolen from its Trading with Schools platform, which was targeted in a cyber-attack in September.
However, the council said cyber security specialists are still working to establish the full scale of the data breach.
Headteachers at the affected schools are now advising families to take precautions, including checking bank statements for unusual transactions and reporting suspicious emails.
Vik Verma, the council’s Director of Education and Skills, said: ‘We recognise that news of data being stolen in connection with this cyber-attack will be concerning for parents, carers, school staff and the wider school community.
‘We understand that people will want to know what information may have been affected and whether it relates to them or their family. Establishing that remains a key priority in our investigation, and officers are reviewing a large volume of material as quickly as possible.’
He said families should remain cautious about unexpected messages, emails and phone calls while the investigation continues.
Cyber criminals have stolen more than 320,000 files from Bristol schools and published them on the dark web (file picture)
‘We would encourage everyone to remain vigilant,’ he added.
‘We continue to work closely with schools, cyber security specialists and law enforcement partners to understand the full impact of this incident.
‘We will keep schools, parents and carers informed as more information becomes available.’
Avon and Somerset Police and the South West Regional Organised Crime Unit (SWROCU) are supporting the council as investigations continue.
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Hackers infected the Trading with Schools platform with malicious software on September 21.
Council officials discovered the attack four days later and took the system offline.
The platform serves 88 schools under the council’s umbrella. Of those, 49 use it only to purchase internet services.
The other 39 schools, whose data has been published online, depend on the system more extensively for daily operations, including scheduling, lesson planning, staff records and payroll.
The cyber incident has caused significant disruption across the schools, with many unable to use their telephone systems while the platform was shut down.
Teachers have also been working longer hours as they try to find alternative ways to deliver lessons.
The dark web is a concealed part of the internet that is not listed by standard search engines and requires specialist software to access.
Dr John Curry, a reader in cyber security at Bath Spa University, said while the dark web has its legitimate uses, it is mostly used by people ‘selling bad things’.
He said that hacks such as this are usually carried out by crime syndicates who extract the data then pass it on to brokers who attempt to sell it.
He told BBC News: ‘They chop it up into chunks and decide who could use this – “if you buy this from us, you could use it to blackmail teachers and parents who have people at the school, you buy this set from us you might be able to use it for financial data”.’
The cyber security expert said once the data has been sold, it is often dumped online in its entirety to show other criminals what the hacking group is capable of in order to garner more work of this nature.
The Daily Mail approached Trading with Schools for comment.
The SWRCU was not able to comment on a live investigation.