Additional assignment-related features and issues within Microsoft Teams for Education
Hi Jaspreet,
I'm Nik, an independent advisor here to assist you with this.
I have gone through the post carefully, and I understand that you want to hold off assignment review marking until ready to public it for all students.
Just to give you a background, when you publish an assignment to a student, teams creates a student copy in SharePoint site with co-authoring. So, when you review student assignment copy and make any changes, It gets updated to student copy in near real-time.
I have tried in my test environment and couldn't find an alternative to this. As this feature is not available today, I suggest you participate in Microsoft UserVoice forum to report this improvement and contribute to product enhancements https://microsoftteams.uservoice.com/.
Regards
Nik