I don't mean to be argumentative, but during my sailing years, the only time I had as much time as 90 days off in a row, I was having trouble shipping out of the hall. . . . I don't know if you are already in school, or are just planning ahead. I have many classmates that went on to grad school, many becoming attorneys, however I am not sure that they were able to do that with a busy sailing schedule. As a 3rd Mate (or engineer for that matter), you will be trying to grab any berth that you can (or at least I did). Should you have a 90 day stretch of time off, it may or may not coincide with a grad school classroom semester. That said, with the option of online classes now, it may be easier to do.
If you are still in high school and weighing your options, I admire your forward thinking. Just remember, a place like KP is not an ordinary institute of higher learning. It is geared to produce Merchant Marine officers for a maritime industry that has changed since the inception of the program. My personal experience is that what I wanted to be and do when I was in high school is VERY much different from what I am and am doing now. . . and I do not regret that at all. . .