Hackers have targeted the private computer systems of six schools on the Isle of Wight.
A so-called 'ransomware' attack crippled the servers of the Island Education Federation.
Such attacks use a computer virus that when downloaded can encrypt data, with cybercriminals demanding money to restore the systems.
At least one school affected has delayed the start to term and said it would take 'months of work' to recreate all the information that was lost.
A so-called 'ransomware' attack crippled the servers of the Island Education Federation, affecting six schools on the Isle of Wight (stock image)
The IT systems were compromised between July 28 and 29 and affected three primary schools, two secondary schools and a sixth form college.
A spokesman for the group said: 'We are working with officers from the police cybercrime unit to pursue the cyber criminals and understand the full impact of the attack.
'There are obviously some significant implications of this, which we are managing and will take measures to secure our systems even further in the future.'
In March, 37,000 pupils at Harris Federation academies in London were left unable to access emails. It was the fourth attack on school trusts that month.
Lanesend Primary (pictured), one of the six schools affected, announced that pupils would return three days later than planned for the Autumn term due to the disruption
It is not yet known what data has been encrypted in the latest attack, but personal data of pupils and staff - including registrations, addresses, and dates of birth - are often stored on servers.
One of those affected, Lanesend primary, has announced that pupils would return to the classroom three days later than planned for the Autumn term due to the disruption.
A statement said: 'Having spoken to our service provider, we understand that the attack means that all the information that we stored with them has been encrypted. This means we cannot and will not be able to access it again.
'As you can imagine, the team now have hours, days, and months of work ahead of them to recreate the information that has been lost.
'In order to assist with this painstaking process, the Trustees have approved the school to close for 3 extra days at the end of the summer holidays.'