Update - this item was completed and shipped on 11/3/2020
When a Student is Dropped from a Gradebook and removed from Google Classroom, the nightly sync process may re-add them to the Google Classroom. We are working on a fix for this issue.
For Elementary teachers that have Inactive students that have left the school, but are still popping back into their Google Classroom; I have gone to student's record and changed the stu.cu (teacher) field from the teacher's name to Unassigned. That stopped it for 6 out of 8 teachers that have complained about this problem.