Purchase, and then Download, the .iso file and convert that into a bootable DVD.
Windows 8.1 Pro for Students
http://www.microsoftstore.com/store/msusa/en_US/pdp/Windows-8.1-Pro-for-Students/productID.288769600
To convert a downloaded .iso file into a DVD right-click on it (use any Windows 8 machine) and select "Burn disk image". Follow the Wizard.
Software downloads
http://www.microsoftstore.com/store/msusa/en_US/DisplayHelpSoftwareDownloadsPage/#troubleshooting
Upgrade to Windows 8.1 from Windows Vista or Windows XP