Dear Mrs Robinson,
Actually, it is my work which brings me to write this letter. You may have noticed my attendance and punctuality slipping the last two weeks. Please do not think that this is in any way a reflection on your lessons. I have simply been inundated with work ever since the marketing manager at the company that I work at resigned without notice two weeks ago. Being the next most senior person in the department, I have had to take on the role in her place. Due to the work load, I am afraid I will not be able to continue your course at this time.
I anticipate that by next semester, things will have settled down, and I will be in a position to re-enrol in your class. I also hope that it will be taught by yourself again next semester, as I find that everything you teach has a lot of relevance to my company.