it all seems to depend on when your spring and winter breaks are. if you have a reallyyyyy early or really late spring or winter break that not many other schools have, then theres obviously going to be less people. usually the same regions of the country have break at the same time. for example.....on my winter break cruise, there were TONS of long islanders.
on summer cruises though, (especially in the middle) you are sure to have a bunch of people. just don't go too early in june or at the end of august...because lots of schools are already back in session/still in session then.