Turnitin Feedback Studio is a mobile app used by lecturers to make Turnitin assingments.
Problem: Error "The class access code you have entered is invalid."
Solution: On the ipad goto Settings.
Then scroll down and choose "Turnitin" (appears on the left at the bottom of the list).
Then choose "Service" (on the right).
Then tick the "Feedback Studio UK" option.
Then log in again to Turnitin Feedback Studio.