With Zearn, student data is maintained within student accounts; this allows students to save their progress and badges from year to year. It is not possible to delete student badges without deleting the student's account.
While teachers can't delete lesson badges, they can reset their students' assignment. Once the assignment has been updated, the student will use their Next Up feed to access through lessons from the assigned point and work through them in order and at their own pace. All badges previously earned by the student for the assigned mission will be visible, but the student will not be able access future missions until they complete the assigned lessons again.