The primary purpose of an Assignment Group is to identify a group of devices to which an app or Profile will be assigned. There are two types of Assignment Groups: Smart Groups (membership is dynamic and varies based on elements of the device or user) and Organizational Units (membership is determined based on where the device exists in the OU structure.
To add, view, or modify Assignment Groups, login to Workspace ONE, click Groups & Settings (in the menu column on the left), click Groups, and then click Assignment Groups. You can create as many Smart Groups as you like for your school. A common use of Smart Groups it to create one group for each app that you deploy. You assign the app to the Smart Group and then you can add only the iPads to the Smart Group which need the app. This prevent you from having to deploy every app to every iPad and saves storage space on the iPads.