In an effort to prevent the accidental deletion of data, we don't allow data to be deleted. Instead, data can be hidden at all layers: class, assignment, question. For example, if you want to hide/unhide a class, select that class, click the pencil in the top-right corner of the screen, slide to hide/unhide and then click the Save button.

Data that is hidden can always be viewed by the teacher who owns the data, but this hidden data cannot be viewed by a student. You can always unhide the data at a later date.