Dear [instructor’s name],
This is to inform you that I am unable to attend classes from [starting date] to [returning date] due to uncontrollable reasons. I will have limited access to email and phone so it will be hard to keep in touch.
I humbly ask for the material outline of the lessons you will cover with the class while I am away. I will do my very best to keep up with my studies so I am not behind when I return.
I am hoping for your understanding and thank you for your help.