I don't know where you are located, but in California, most employment situations are "at-will", which means that either party--employee or employer--can choose to end the relationship at any time for any reason. With that being said, I would seriously think twice before I made any long-term decisions regarding this particular job for one big reason: Your supervisor is demonstrating his willingness to work with you to improve yourself by allowing you to take a leave of absence rather than forcing you to make a decision about your job.
I don't know what industry you are in or what your job duties are, but what he is doing is literally holding your space for five months while you finish your studies. That's five months he is choosing to be shorthanded, while also forcing your coworkers to add on to their load to cover your productivity. I understand you may be unhappy with your current salary and responsibilities. After you are done with school and you return to the job, you may want to take some time to consider what sacrifices were made to allow you to finish, and, at the very least, sit down with your supervisor and have a meaningful discussion about your future growth with the company. You will have your degree, which automatically increases your marketability. What you also have is knowledge of the company, which has value in and of itself. Any new person coming in is going to have learn that which you already know, which has a cost to the company.
My suggestion: finish your program, get your degree, return to the job, and assess your options then. If you still want to leave after you've talked to your supervisor, your conscience will be clear and you will be able to tell any new potential employer that you are seeking challenges commensurate with your newly acquired education.