I did this once in HS, but it was for an english exam.  I forgot I had an exam that morning.  The teacher called my home a little after 9am when I didn't show up.  Seeing how I didn't care it was only english, I took my time to get there.  When I got there, had to read the exam several times and still didn't get it (along with the many in the exam room who've asked for help), so just took off when the hour was up and didn't do any of it.  I knew what I was walking in with, so I didn't care.  Yeah, that was my HS days 

  I could be young and irresponsible.  It's my right, dag-nabbit! 
For teh same reason of afraid I may not wake up in the morning, I DON'T sleep on study nights.  I study on and off throughout the night.  I do well in tests using that method.  Works for me 
Goodluck fella.  If you did infact put in an honest effort in class, participated in discussions, did well in tests and such, you should have no problem getting authorisation writing the missed test.