Canvas, a learning management system used by thousands of schools and universities, was knocked offline by a cyberattack on Thursday, preventing scores of students from completing assignments and studying for final exams.

Canvas, operated by parent company Instructure, is widely used by schools to manage grades, assignments, lecture videos, course materials, and communication between students and faculty. The hacking group behind the breach said nearly 9,000 schools worldwide were affected and alleged that billions of private messages and records had been accessed.