Hello Jon-Paul,
Below is documentation outlining how to delete a member from your organization.
Manage members—ArcGIS Online Help | Documentation
Within this article, the following information is presented. Depending on how the member was created/added to the organization, the account may or may not be able to be used as public. Use the article and information below to determine this for yourself.
"Deleted members who were added automatically or joined with ArcGIS accounts that have not been enabled for Esri access are removed from the system and can no longer be accessed, even if they were established before joining the organization. Accounts that have been enabled for Esri access and existing Esri accounts that were used to join the organization are still available to use with Esri web resources such as My Esri and GeoNet. They can also be used as public accounts (but they do not have access to any content or groups that became associated with the organization)."
Keep in mind before deleting a member any licenses they have must be revoked. Also, their content and groups must either be deleted or transferred. It sounds like you are wanting them to retain this content, which can be done by following the instructions at the URL below. This outlines how to use ArcGIS Online Assistant which allows you to do exactly what you were asking for, have students retain their content.
https://community.esri.com/community/education/blog/2017/11/07/transferring-student-content-arcgis-o...
Hope this helps!
Lukas
