Hi! As far as I know, Khan Academy doesn’t currently have a built-in option to print assignments or quizzes. These activities are designed to be completed online, so there isn’t an official printable version.

If your goal is to reduce guessing, you might consider having your work on paper before entering the answers. Also, you could copy paste the document on the lessons whenever you meet "reading" lesson on Khan Academy. My suggestion is screenshot the quizzes and put them into a google doc and separate them by content so that you can review it whenever you need them. 

If having printable assignments would be helpful for your classroom, you could also submit it as a feature request in the Support Community so the Khan Academy team can consider adding it in the future.