In your Teacher Control Center, you can reset any student grade and/or time on task that you like. Just log into your Teacher Control Center and follow these steps:
1) Select the Monitor tab
2) Select the appropriate Classroom and Activity
3) In the Panel marked Students on the left of the screen, select the student's ID
4) In the upper right hand corner of the student page, select Manage Grades
5) Scroll down until you find the quiz (or other activity) and click its "checkbox"
6) Select the bright red Delete Checked Grades button in the upper right hand corner of the screen