If the student logs back in with the original join code or link, they will begin a new session. To rejoin the same session, the student needs their unique join link given by the teacher. You can access this link by clicking the three vertical dots under 'Actions' on the student's row and then selecting 'Show Join Info.'

This is the only way for students to rejoin the room unless your school or district has a partnership with MagicSchool and uses SSO.