yes I agree with ehBeth. I think it would be different if you planned the cruise with one other person where they would have to go alone. But this sounds like a group of friends that are still free to travel without you. Not to mention they were tagging along and inviting themselves along with you.
Just be yourself and say you are really sorry and did not foresee your education getting in the way. That you would not have canceled except for the fact that this is terribly important to you.
Good friends would understand - those that would not - you might be better off without.
I once had a friend that did not understand when wedding plans got in the way of me meeting her for coffee (yep it was just coffee) - I explained that my future marriage was very important to me similar to when she canceled on me a few times when she said she had to study for her law degree. I mentioned how I understood that. Her reply was that was very important!
You know what this woman is no longer a friend (not because of this particular incident - but another) - her reply to this should have been an indication as she only got worse.