Hi Angel!
First of all, Congrads grad! This is your day and you deserve it! I remember my graduation day back in 2003. It was a hot, sticky long day and I felt the same apprehension you do. Our ceremony was three hours long and it was tough, but I did it! You can too!
Try and think of this time as a "meditation time" Sounds ridiculously corny but it might help. Just imagine that everyone cannot see you (don't strip or anything ;) ) but try and relax into your seat. No one can tell if you are doing a progressive muscle relaxation (lightly tightening the muscles in each part of your body then releasing them and enjoying the feeling) You can do this slowly for the whole time if you wish.
If it is anything like my grad, the speeches will be boring so you won't be missing anything if you "veg out." Try and think of this as the very end of your high school experience. You are exhausted and now you get to sit and relax and be excited that you have completed what was probably the most challenging experience you have ever done. (If you were anything like me in high school, you were a friggin basket case with anxiety and with the addition of other "normal" high school stresses."
Enjoy your day. Have a nice breakfast with friends or family and enjoy the graduation festivities. Remember, if you have to leave during the ceremony, don't worry about it. People will just assume you have to pee or something. Two hours is a long time to sit if you have been drinking (soft beverages of course ;) ) during the day and celebrating.
Don't worry and tell yourself that this one day is yours, you deserve it and you will be fine!
I'm thinking of you and hope you have a fantastic graduation!
Jen