You better have a REALLY good excuse if you can't show, and/or at the very least, call them! Penalty is up to, and including jail time (at least here in Ca it is). Not likely, but should be clearly stated in your paperwork. Read your paperwork very carefully. If you have a GOOD reason, they will likely just reschedule you.
Oddly enough, I had to show up today, and I am waiting to find out if I will be dismissed or not.
BTW, this should be in the After Hours Lounge, not here.