According to a chart on the Apple developer page 46% of iOS devices were already using iOS 8 as of September 21 .
The vast majority of the remaining users are on iOS 7, with only a handful still using earlier versions. Users not upgrading from iOS 7 likely include those with older devices and those with little memory for over-the-air updating. For those short on device memory, the update can be also be installed using iTunes and a USB connection.
Connections instructors will probably see a mix of devices in classrooms for a while yet. The new course manual for iOS 8 will be published shortly and will still cover the features in iOS 7 with some changes to the screenshots and to iCloud. I would recommend that, where possible, instructors contact students ahead of time and find out if which iOS version they will be using, and if they plan to upgrade refer them to the link above to have the upgrade in place prior to class. Students continuing to use iOS 7 will still get a lot out of the class, they will just be missing all of the new features and enhancements the new version offers.