Well you've definately come to the right place!! I know that when i was in school I definately could have used a lot of these.  First up, this article has some great real excuses that you can use in life, and below that, this article has some links to some of the best sites on the Internet, for the more funny and less practical excuses.

We have one choice so far.  In the future, I'll certainly add more.  If you know of one, by all means post it in the comments section.

Get your hands slighty wet with hot water, just enough so that they're damp. This will make them clammy and will confirm your mother's intuition if she holds your hand. Then, for about ten minutes, do some coughs and deep gurgling in your throat so that it scratches a bit. This will cause you a bit of discomfort, perhaps, but it will make your throat look inflamed temporarily. After all this preparation, go tell your mom that you're sick, and when she takes a look at your hands and throat (as most mother's do) she'll see that you're telling the truth and BOOM no school for you. A perfect ready-made excuse for missing school.

There are two excuses so far for this one.  Definitely take a look at them as they seem pretty efficient. 

1.  Tape a piece of gauze (that cloth stuff that the doctor uses to do bloodwork) to your arm. It should be in the crook of your arm. Then, if your teacher, or whoever else asks why you're late, say you were just at the doctor's office to get a blood test, but you forgot a note. Since you still have the gause taped to your arm, noone will think twice to question you.

2.  This one is one of my favorites. This is if you get to school late and a teacher or principal catches you. Before doing it, get your cell phone a new voice machine message. Put it right up to your regular phone with your parents answering message, and record so that they are the same. Turn off your cell phone. Then, when the teacher/principal catches you, tell them loudly that you were with your parents.

When they try to determine if you're telling the truth, repeat your statement, daring the principal to call your parents. The principal will quite likely end up calling your parents; give him your cell number when he asks. He'll then call it and leave a message for your parents, on your cell phone for you to delete.

"I had bad Diarrhea" - Noone's gonna second guess you if you're being this blunt.

"I got into homeroom late - my teacher must have missed marking me down" - Simple and hard for him to verify usually.

"I walked here to try and be green [with smile] Shouldn't our school emphasize being green, [Sir/Maam]?"

More to Come!! - Updated Frequently