Baltimore County Public Schools is in charge of all public schools in Baltimore County, Maryland, and it is the 25th largest U.S. school system. District-issued Chromebooks were likely not affected in the attack as they are powered by a read-only Gentoo Linux-based operating system and use a cloud-based file storage system that blocks such malware from encrypting their contents. The district also had to shut down most of its network due to the large number of systems impacted during the incident.
Source: https://www.bleepingcomputer.com/news/security/baltimore-students-told-to-ditch-windows-pcs-after-ransomware-attack/