TL;DR
Teacher identity, school permissions and section teaching assignments serve different purposes. Check all three.
Steps
Open Teachers in Insights and select the teacher.
Verify the school email and access level with an authorized administrator.
Open the relevant sections and check the default teacher and teachers assigned in schedule entries.
For co-teaching, include each intended teacher and verify their access to the section.
Administrative access alone does not make a person the teacher of every section. Co-teachers can be affected by changes to shared published work and linked copies. Agree on responsibility before editing or unpublishing shared assignments.
Related help
Reuse assignments across sections · Publish assignments with useful timelines · View an absent teacher’s schedule