Grade forgiveness allows students to retake a course in which they received a low grade, and the most recent grade or the highest grade is the only one that counts in calculating a student’s overall GPA.
解析：
本句是一个并列句，由and引导，前边的主干是Grade forgiveness allows students to retake a course， 后边in which引导定语从句，修饰 先行词 “course”; “and”之后的分句，主干为the most recent grade or the highest grade is the only one， “that” 引导定语从句，修饰前面代词 “one”。
翻译：
成绩宽恕使得学生可以重修得分较低的课程，最近一次成绩或者最高成绩就成为衡量一名学生GPA(绩点)的唯一重要依据。
