School districts that were affected by a PowerSchool data hack in December are now facing extortion attempts by cyber criminals, according to the K-12 ed-tech company.

Cybercriminals demanded ransom payments from school districts nationwide this week, using millions of K-12 students’ sensitive data as leverage after the files were stolen from education technology giant PowerSchool in a massive cyberattack late last year.

Software maker said it paid its attackers even as the true size of the theft is still unknown.