GitHub Education offers a variety of tools to help educators and researchers work more effectively inside and outside of the classroom. More details are available at https://education.github.com/. GitHub Classroom is open-source.
A GitHub Classroom assignment invitation link can be accepted by anyone who has the link, and accepting it associates that account with the assignment. This is intentional, because teachers share these links through their own channels and we cannot know which accounts are meant to receive them. Treat an invitation link as something to share carefully. Reports that an unintended account can accept a shared link are ineligible.
GitHub Classroom acts on an organization the teacher owns, so it can create repositories, manage teams, and add or remove members within it. Reports that a teacher or organization owner can perform these actions, or that removing a student from a classroom does not immediately revoke every prior access, are working as designed.
Repositories created by Classroom follow the visibility the teacher selected. Assignment content, submitted code, and roster identifiers in a public repository are public. Reports that this content is readable are ineligible unless you can show a private repository being exposed.