Did they know you had these exams? Did you tell them you needed time off for them? Did they hire you knowing your students position?
If the answer is yes ... forget, cool off and chalk it up to experience.
If you never told them, then I'm afraid to say, it's your funeral, mate! Again, chalk it up to experience and make sure you keep everyone informed next time.
Only hearing your side of the story, makes me think that it's an excuse to get rid of you, as skipping three times to be fired is pretty draconian. Again chalk it up to experience.
Don't take it too hard, find another job. Sounds like heartless words, eh, but it's what you have to do.
Oh yes ... talk to you boss about it too, if it's just an e-mail out of the blue, it could always be a practical joker!
I actually worked at one place where the VP's second in command kept sending "love" e-mails from her account to another employee. He eventually turned up for a romantic evening with her ... she didn't know ... more to it than that, and quite funny in the end, but you get the drift.
