During Add Item
First, make sure you have folders ON for your class. Click on Your Profile > Manage Class > Show Add to Folder Step and make sure that says either Teachers or Teachers and Students.
Then if you want to add an item to your Private (Teacher Only) Folder when creating a new post, you can do that on the Folders selection screen. Tap on the small blue lock icon on the bottom right corner of the folders selection screen, then make sure the Teacher Private switch is green. Then hit the green check to finish uploading the item. The item will not be visible and no notifications will be sent to to students or connected parents.
From the Approval Queue
You can add student work into your Private (Teacher Only) Folder from the Approval Queue as well. This might be useful for assessments your students complete that you don’t want shared with students or parents just yet. Tap on the folder icon below the item, then tap on the Private (Teacher Only) folder to add it to the folder. When you approve the item, it will not be visible to students or parents.