Students need a stress-free zone for learning grammar, and the choice to revise a writing assignment gone bad (where the writer really didn’t initially understand the concept) is reassuring. Thus, during or after a class discussion, I encourage students to revise their incorrect sentences. They may revise in class or until 11:59 on the day of class; if they revise after class, they must send me the revised paper electronically on the same day. Sometimes I allot five to ten minutes at the end of class for revisions, depending on the difficulty of the assignment.
Thus before I grade a homework exercise, it has usually been revised, sometimes extensively. But students may also revise after their writing assignments are graded. Indeed, I require that an assignment graded below 75% be revised, without penalty, within a week of its return. I do not, however, accept revisions of graded papers where the student has scored 85% or better. That student has already demonstrated an acceptable comprehension of the topic.