英文请假邮件

大意是：
xx老师（男），因为XXXXX的原因，我明早不能上你的课了。

1楼以为我看不懂英语吗，不要乱答！
我主要不知道邮件格式是怎样的。
老师的称呼可以直接就用个“Sir”吗？
Subject那一栏通常填什么？
说完不能上课后要说些别的客套话吗？

Dear Mr. xx (i think Mr will be fine, or if you are an uni student i think u can just call his name?)
i couldn't attend your class on tomorrow due to xxxxxx(reason), will be seeing you next time.
thanks

(that should be ok XD)

dear sir,

can i please ask for a leave on 8th june morning because i have something emergent to deal with and i need to go back to school to handle it, could you please grant it?

thank you in advance.