Hi @FH_CHIU,
When you remove a class, you remove all the assignments - if the original classes are not there, then the associated assignments are gone.
Before removing a class, it’s good practice to ask students to copy their assignments to Free Play, or do this yourself and put assignments into a folder.
An alternative is to Archive a class. See What is the difference between archive and delete class - #5 by Pedro_Cruz for a good discussion of the differences.
Hope that helps! Let me know if you have any further questions about this. If this answers your query, please mark this post as the Solution.
Many thanks,
Geoff @ TechLeap