Schools and universities across the country are recovering from an outage that knocked down Canvas, an online platform that manages exams, course notes, lecture videos and grades. The disruption tied to a cyberattack hit in the middle of finals period for many colleges, a high-stress time when students and instructors rely heavily on the platform. By late Thursday, Instructure, the parent company of Canvas, said the platform was available again to most users. The hacking group ShinyHunters claimed responsibility for the breach, said Luke Connolly, a threat analyst at the cybersecurity firm Emsisoft. On Friday, Instructure and Canvas no longer appeared on a site where ShinyHunters lists its targets. Some schools, however, have continued to block students and teachers from accessing Canvas, citing an abundance of caution while assessing security threats. Here's what to know about the outage. What is Canvas? Schools and universities use Canvas to manage nearly all aspects of instruction.…