When school computers starting acting strangely, Cloquet district administrators knew something was wrong. When a computer help icon offered to fix the problem for $6,000, they knew they needed to close schools Thursday.

"So many of our systems are tied up with technology anyway, it would be very difficult to run school," said Ken Scarbrough, superintendent for the district.

The district shut down all five of its schools. Students returned to class Friday, even as every computer in the district was being repaired, except for iPads and Chromebooks, which were not affected by the phishing scam. That left teachers finding new methods to teach their tech-oriented curriculum.

"People said, 'OK, we can do this,' and then they just make it work," said Matt Winbigler, the district's technology integration and instruction specialist.

No private student or staff data were affected by the virus, Scarbrough said.

Officials called police as well as the FBI. The FBI is now working with the district's technology team.

Besides being instructional tools, computers control numerous school systems affected by the virus, including the air conditioning and lunch number systems. Now, the district must take lunch numbers with pen and paper.

In order to shed the virus, the district rebuilt its servers and began reformatting its computers, a process that could take weeks.

Scarbrough said district devices started behaving strangely Monday, and eventually icons appeared on the screen offering to solve the computer problem in exchange for a four-figure fee in bitcoin, a digital currency that is often untraceable.

The virus could have come from various sources, Scarbrough said, but he doesn't think the malware was specifically targeting the district.

Winbigler said many teachers have offered to help, and members of the tech team and others worked over the weekend to get more of the computers back to working order.

Staff and students will not be allowed to access the computer system until reformatting is complete.

The North Branch Area School District was also struck by a computer virus Wednesday, when a staff member whose computer had been infected accessed a file shared over the school network. Students were on spring break, and the virus was caught very early, so they were able to take precautions.

"We just took preventive measures, so ours was very limited," said Superintendent Deb Henton, adding the district used the opportunity to inspect other files to make sure they were secure.

Ben Farniok is a University of Minnesota student on assignment for the Star Tribune.