Quan,
In Admin>Settings>Reports
Run the Provisioning Report, selecting Users and check the box about 'Include Deleted Objects' at the bottom.
When the report is complete, download the file, find the delete user in the file and change the last column from 'deleted' to 'active'.
Upload the file in SIS Imports.
All the student/teacher work should then be restored.
CAUTION: any time you deal with uploading a csv file in SIS Imports, always do everything in the TEST environment ( https://_______.test.instructure.com ) until everything works as desired and you feel comfortable with the process. Then repeat the entire process in your production environment. (in this case you will probably see that the user has not been deleted in the test environment, yet. It's still good practice.
If you would like someone to walk you through the process, please call the support phone number. They would be happy to help you with this. You can also refer them to this post.
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