Hello
I’m Adeyemi and I’d be happy to help you with your question.
If you're seeing former students in new assignments, it could be due to a delay in the system updating the changes. You could try manually restarting the Teams app.
Unfortunately, as of now, there isn't a direct way to remove a student from an assignment that's already been set up. If you want to remove a student's work from an assignment, you can go to the File tab in the team, choose Open in SharePoint, and navigate to Site Contents and Student Work. Here you will see two folders: Submitted files and Working files. You may delete previous students’ work from these folders
These references can help too: https://answers.microsoft.com/en-us/msteams/forum/all/removing-students-from-assignment-lists-that-are/663890f8-1a15-4019-a13d-e25ec8dfd7f8. https://answers.microsoft.com/en-us/msteams/forum/all/remove-the-materials-from-assignment-section-which/f9c18e18-9ea5-401a-aa4e-a0ad0eaafbd1.
I hope this helps
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Regards Adeyemi