dear ms.xxx，
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 grateful.
earnestly yours，
xxx