Teachers can only be a co-teacher in another teacher’s classroom by receiving an invitation from that teacher or by being assigned as a co-teacher to the class by admin.
Have the teacher of the class follow these instructions to add you as a co-teacher.
Co-teachers can view student data (but can't modify it or the module setup) for any classroom at their school with permission from the teacher of record.
Individuals with admin access have the ability to add themselves as a co-teacher via the school/admin portal or to assign co-teachers.