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While I can completely understand your frustration with this situation, it is not one in which you would typically have any legal rights or protection. This is simply because the law does not require employers to accommodate an employee's school attendance. The law also does not require employers to be fair or reasonable. Absent an express agreement to the contrary, employment is "at will" and thus terminable or modifiable at any time for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination/modification is fair, reasonable or even true.
So, the situation you find yourself in is just not one in which the law provides any guidance. As unfair and ridiculous as it is, your employer is free to get mad at you and even retaliate against you due to class conflicts, even if you explained these conflicts weeks in advance. All you can attempt to do is manage this situation on an interpersonal level. This means continuing to do what you have been doing, by warning your employer as much in advance as you can about potential conflicts, and trying to avoid conflicts as much as possible. It is an unfortunate reality that often times people trying to better themselves by obtaining an MBA or other advanced degree do put a strain on their existing employment relationship. Ultimately, if you have to choose between the degree you have invested so much time and money into and the job you are presumably trying to move up from, I think for most people the correct choice will be clear.
I hope that you find this information helpful. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
If you do not require any further assistance, please be so kind as to provide a positive rating of my service so that I may receive credit for assisting you. Very best wishes moving forward.