I am not sure that the essay I write below is totally grammatical. Can you give me any advice? Thanks in advance! ^^

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For instance, my friend jack, good at physic, got a low grade of his physic class last semester due to an accident. There were only two big exams per semester and the grade of each exam took fifty percent in the final grade. Before his first exam, he went swimming with me. Unfortunately, he got the flu the next day. Thus, during the exam, he got a headache and he couldn't concentrate on the exam. He was so disappointed that he got a low grade for the first exam. Even though he got a high grade in the second test, he still got a low final grade. He explained to his professor and wanted to have a chance to have another test to approve his ability of mastering the knowledge well, but his professor said it was unfair for other students. He deserves a high grade because he is adept at physic and he is diligent. Students' grade is determined by a big test is not reasonable.