Pupils at Dorchester's Thomas Hardye School return to classes and exams this week – with no certainty that IT systems will be working following a cyber-attack.

The school was asked for an undisclosed ransom sum to unlock its systems after the attack, a week before the half term break, but refused to pay.

If the attack results in a complete loss of data, the school's headteacher has said says the implications could be "far-reaching" for the school.

All data on the school network was encrypted, together with the back up system.

It is still not clear whether personal data held on the school system relating to more than 2,000 pupils, carers and staff has been compromised.

A notification on the new school website, which IT experts have managed to get running, warns that systems are still not working properly.

Said a message to parents from headteacher Nick Rutherford: "Although there is still a lot we do not know, it seems to have been a sophisticated attack, launched at a time when the network was backing up to the off-site server, which was therefore also encrypted.

"Possible outcomes range from a best-case scenario where most or all the data can be recovered, to a worst-case scenario where none of it can be recovered. If it is a complete loss of data, then the implications will be far-reaching."

Although examinations have been continuing as planned, some pupils did have assignments required for BTECs and A-levels saved on the network. The school says it will be working closely with exam boards to support those students on the return to school after half term.

Said Mr Rutherford in a message to parents and carers: "As well as the impact on the school, the attack may also constitute a data breach. We hold a large volume of data on staff, students and parents and are aware of the seriousness of any such data breach.

"We are following guidance from the Information Commissioner's Office and we will continue to keep you updated once we have further information as to the level of risk to personal data."

He said that even if data recovery is successful, there will be an impact on the day-to-day running of the school as the network is rebuilt, including the need to re-create teaching resources and repopulate the school's management system.