Dear Ms.Riddle，
I would like to know if I could ask for a casual leave of absence from your Oral English class this Wednesday morning.
Yesterday evening I got a phone call from my cousin, who is now running a small firm in suburbs of this city. He needs an interpreter for two days but cannot find the most reliable person, so he wants me to have a try. Personally, I regard this as a golden opportunity to put what I have learned in your class into practice, but I need your permission of absence. If you think I may go and help him get over the difficulty, I am confident that I’ll do a good job and both of us will be grateful.
Thanks. Your kind permission will be greatly appreciated.
Earnestly yours，
Li Hua