Hi arthur-82. This is a difficult one because it is hard for me to know exactly what the business relationship is between you and Joanna. How formal it is, the difference in seniority, that sort of thing. Also, the second paragraph is a little confusing in that it suggests you were earlier offered the sort of arrangement you now desire, as in: "there is a possibility that you agreed to me" and "I would be very grateful if you consider this option again". But I'm not confident that that is what you're saying. Your very last sentence is also a puzzle because I don't know if "ask for an exemption" refers to the arrangement you have been suggesting in your letter, or is another course of action entirely. Anyway, I'll have a go at tidying it up for you.
In response to your letter of 15 January I would like to explain my current situation and how it affects my work. Currently I am studying Marketing and Management at the University in Warsaw. It is the final year of my undergraduate course and as I am preparing to defend the work I need more time for study. Furthermore, on 27 January my exams begin and Iíll also be starting graduate study during that month. All of these obligations mean that I need more study time and I can only find such time in the evenings.
In the five years we have been working together I feel I have always been loyal and cooperative. At the time I began the work I did not know if I would be able to study as well, but my situation has changed considerably and I do not want to lose this chance to improve my skills.
Therefore, I would like you to consider my not working the evening shift from 21 January to 31 March. From 31 March onward I would again be free to resume the evening shift as per our earlier agreement. I hope an arrangement can be made that is satisfactory to us both, otherwise I must apologize for asking for an exemption within the statutory term.