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