You may need to have them switch their roll to students before they click the quiz link. What is happening is the teacher role allows previews (you could alternatively turn this off, since the idea is to let them do some teacher things but not all). But that doesn't solve the immediate problem...if you id the quiz specific identifiers and go into the background data, you can find those attempts and change the marker from preview to not preview. (I tend to just look for the preview quizzes and switch them to attempts and not bother trying to figure out which students they apply to...but you may want to look at student id as well to ensure you are choosing the right attempts). This is a bit dangerous (mucking about in the data always is) but will not require them to go in and retake the quiz.
Maybe I should repeat...dangerous...if you fiddle with background data, be careful, you can do some truely heinous stuff to your users/courses/site when you do this. Be really sure before you do anything...