Grammatical doubt

Dear all,

Kindly request you to correct the grammatical errors in the following letter to the Principal for remission of a fine. I am given only the body of the letter.

"Yesterday I reached the school late by five minutes and was fined Rs.200/- by my class teacher.

Actually I had reached the school on time. I was detained at the gate for about ten minutes. It was because frisking of students of 10th who had to appear in the Annual Board Examination was going on and I had to wait I was not told by the security guard that the back gate of the school had been opened for students of other classes. I Will therefore, be grateful if you remit the fine imposed on me for no fault of mine".

Thanks in advance