If I had a choice between a 3 night cruise or none at all, I would definitely take the 3 night. ANY cruise is always better than NO cruise! However, if you can swing the 4 night, it is amazing how much better and more relaxed that extra night makes you feel.
As far as cruising on school days....I think that depends a lot on the age of your children, your school's policy and how well they do in school (i.e. how easy will it be for them to make up for lost time?). For example, we took my daughter out of school for vacations when she was in elementary/middle school and she had absolutely no issues (we did take some homework along on vacation when she was in middle school); but when DD was in high school it was MUCH more difficult for her to do the homework ahead of time & to miss the classroom lecture/projects. In every instance, we approached the school and her teachers WAY in advance to tell them about our trip and ask for teachers to either assign homework early so she could work ahead and have it done before we left or to allow the homework to be completed during our vacation to be turned in upon our return.