How would I leave my employer gradually mainly due to school for advanced degree?

I have been looking the online forums and blogs about "leaving employer after greened" and I have a good grasp of what is expected. However my case might be a little skewed off of the "bell curve" since my reason of leaving would associate with "advanced degree". To Tell the Brief of my case. EB3 RN, took 6 years to get greened. Now, more than 1 year lapsed since. I am starting my doctoral level education this fall and will likely drop down work hours. I may have to resign due the load of the school work (if that will be the case). But, I will try to work at least a little bit as much as I can tolerate both school and work. Now back to my question, would I have an OK reason to leave my job with my specific circumstance?