Dawn and Mike,
Don't despair just yet! As a former school teacher, who loves to cruise, I can totally empathize. I had to bite the bullet and pay premium prices to indulge my passion. However, I have discovered some tips that might help. First get in touch with a cruise travel agent who is very knowledgeable. Ask if he/she is part of a large consortium like Vacation.com, AmericanExpress or Virtuoso. If so, they often have group space on numerous Christmas sailings that make the cruise more affordable. It may still cost a little bit more than off-peak deal rates, but still won't break a teacher's tight budget.
Secondly, if that fails and you still can't find anything for the Christmas holiday - opt for a Spring Break cruise - I know there is availability on a number of Spring Break group pricing cruises right now. And last year, even though this may not hold true this season, there were huge bargains being offered by cruise lines for individual bookings even during Easter Week.