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Becuase Senior Three School year in our school is just for the review of college enterance exam,there isn't any new lesson,and I have already taken all the lessons.

As for the third year in our school just a review of preparation for college entrance examination ,we have no extra subjects ,and we had learned all the classes .

Because of the third year our school year is just to prepare for the entrance examination review phase, no new courses open, and I have finished studying all the courses.
  

The third year study in our school is just a revision process required to make a good preparation for college entrance examination, therefore no new subjects is offered. Plus, I have finished learning all the courses.

The last year of my high school is just a review period of College Entrance Examation,so no new course is offered and I have already finished all high school courses.

简单，明了。只用了SO，应该会稍微缓解前面句子太冗长的问题吧

加油吧 祝你成功！