write a formal letter explaining why the I-20 date wasnt extended. explain that i was still enrolled in classes and  had 12 credit hours. i didnt check the dates due to which the I-20 wasnt extended and how terribly sorry i am and have worked very hard the past two semesters to get my grades up and from now onwards will make sure to check the dates and be more carful about it.

the letter should be two full pages and explaining the situation and showing how extremely sorry i am and that this wont ever be a problem again.