As irritating as it is and I can’t tell you how to feel or if you should be mad or not. But I think you can logically figure out in your situation where it is difficult to find a job, you should accept your supervisors answer. Study for your finals on personal time (before/after work, lunch hours etc) knowing that with your newly acquired degree you can be in search of a better job…maybe you can one day be HIS boss.

Good Luck on your finals.

Ask her for the reason. Tell her that you need that time to study both for yourself and for the benefits of the company. If you had your study approved by the company with reimbursement, there is no reason why it could be denied.
Keep the dialogue in a calmly manner. Madness or rage goes nowhere.
Keep pushing your points.