A student might request to have their progress reset because they intend to take a course again, or for another reason. You cannot reset progress, but you can make sure all attempts prior to a certain date (the starting date of the class) are ignored.
To set the start date of the class, go to Teacher > tab, Settings, Start and end date.
Here you can choose the time period that will be taken into account when showing student results. If the end date has passed, students will see this notification on the home page: