My understanding is that you can bypass one class date.
AN EX-military type in my class was called on a Wednesday and offered a class on Monday if he wanted it. He was told since it was such short notice he could of course bypass this offer, AND the next one. He Was there on Monday. My feeling is if you have some sort of obligation, they will work with you at that point. YMMV.
BTW, everyone in my class got a "you've completed phase II" email. Evidently the company stepped up its communications effort last fall, including sending a contact email for status questions. I didn't feel the need to contact them. I was in the pool 7 months from interview to class date. I think it's much faster than that right now.