It sounds like you have done everything you can to make this change palatable to your employer and to yourself. The only thing I see that your employer not liking that seems reasonable is your not being at work during your 'designated' work hours, even if that is a small amount of time. Like you say, what you do on your own time is your business.

Following a dream sometimes means giving something up to allow room for the new passion to grow. If your employer continues to be disgruntled, it will put a damper on your school efforts and you might end up not happy in either the new place (school) or work.

Is is possible to find work that would better suit your school situation? Maybe your current employer has other positions that might work or even give you reduced hours in your current job.

Best wishes - it is a tough spot to be in but you sound like the sort of person who will find a way to work this out.