Hundreds of Norfolk schools were urged last night to double-check their computer security after five high schools - including one at Thetford - had their systems infected by a virus.

Hundreds of Norfolk schools were urged last night to double-check their computer security after five high schools - including one at Thetford - had their systems infected by a virus.

Thousands of pupils were unable to access important online work after the invasive programme breached online security and quickly spread through the ranks of computers in hundreds of classrooms.

Teachers and software experts had to work around the clock to get rid of the Downadup/Conficker worm, which has the potential to get hold of sensitive information and circulate it.

The worm - which also hit Norfolk County Council's professional development centre and sensory support team - has infected millions of computers worldwide, including systems within the French air force and navy and the US air force.

The council was able to cut off the schools and the two other areas from the rest of the network before it spread further, but it is not known whether any top-secret information was compromised by the attacks.

Downadup is believed to be such a threat that software giant Microsoft has offered a $250,000 reward to anyone coming forward with information that leads to the capture of the person who wrote the programme.

The high schools affected are Caister, Wymondham, Thorpe St Andrew, Oriel at Gorleston and Rosemary Musker at Thetford.

At Thorpe St Andrew School, which is one Norfolk's biggest with 1,700 students, the worm was first detected on February 27. It is having an impact on learning because students cannot get access to their work on the system.

Staff and computer experts have been working around the clock, seven-days-a-week to clean up all of the 1000-plus computers at the school.

Headteacher Ian Clayton said: “It's proving really quite problematic. It somehow got through the county's checking system and is causing major problems.

“We have to clean up each machine. We have one computer for every two students and all the staff have a computer. It's very time-consuming.”

He said the school was advising students to not use memory sticks to transfer their work from home to school computers, in case the problem spread.

The worm gets into computer servers and identifies private and internal passwords before taking over the system - possibly in a bid to either send unlimited “spam” emails or to access private information including credit card details.

Stuart Parvin, technical services manager for the council, said the five schools managed their own ICT security. Three had since decided to buy into the council's ICT support, which is available to all Norfolk schools, he said.

He added that the schools now had “fully functioning” internet and email access, and said: “The virus was brought onto the schools' systems via an external source brought into the school, such as by a pen drive, not via the council network.

“We have written to all secondary schools and primary schools whose ICT is not currently managed by children's services to ask them about the level of security that is applied on their networks to gauge the standard of protection they currently have and advise them accordingly if their protection level is not adequate for current and emerging security threats.”