Apple iCloud storage is used to back up the iPad AND to synchronize documents, photos, files, etc. among various devices.
By default every Apple ID gets 5 GB of free storage space in iCloud but this has been proven to be insufficient. So beginning the 2016-2017 school year it is now required that students subscribe to the 50GB storage option for iCloud for $0.99 per month.
On your iPad (or other Apple device with the same Apple ID) go to Settings > iCloud > Storage > Buy More Storage. For more information about iCloud storage plans click here.
You can also check how much storage your iPad is taking up on the device and in iCloud by going to Settings > General > Storage & iCloud Usage and selecting Manage Storage under the section you wish to check.
If you have any questions about this process please contact Tech Support.