It's a really hard decision to make, and I speak from personal experience. When I interviewed for my current position (pharmaceutical industry), I decided it was important for me to be upfront about my desire to return to school so I told them blatantly that I would only work for 1 to 2 years. Nevertheless, they hired me.
I feel reprecussions for being so honest...everyday. Especially when fall rolls around, or when I ask for personal time off, I get -- I wouldn't go as far as hostile -- but a not so friendly vibe from my boss and coworkers. They tend to like to find mistakes in my work during these times, to get me to sweat a little I guess.

Also, I feel I do good work, but I constantly feel my work challenged as a way to discourage my believe that I am an intelligent person. I guess I'm a bit soft in letting them jerk me around, but I never expected them to have the mentality that they could perhaps "assimilate" me.
I have coworkers who I know have strong aspirations to return to school, but they are very discreet. They tend to have a better time than I do.
I guess it is all relative to your situation, and I am just offering my 2 cents as a cautionary tale. Sometimes, the right thing is not the best thing.