Additional team and channel-related features and issues within Microsoft Teams for Education
Hello Daphne,
Regarding to your requirement, you can add the folder back by following the steps below:
1. Go to Files tab in your channel.
2. Click Add cloud storage and select SharePoint > Class Materials.
3. Click Create. Then the Class Materials folder will come back.
If you need further assistance, please feel free to let me know.
Regards,
Seven