It's still very early in Japan (as in 5:30 AM) so we do not have official confirmation from UFA staff, however it has been published in a few different Japanese newspapers that Maeda Yuuka will graduate on December 31st, 2011.
I normally don't post rumors and wait until we have official word, but 5 different newspapers are reporting the same thing and S/mileage is trending in Japan at the moment, so it's a good chance that this is true.
It's very unfortunate news not only for S/mileage, but also for Hello! Project because Yuuka definitely is one of the more talented and popular members (not to mention one of the younger members). It's going to be very difficult for S/milege to continue on after this; leader DAWA is going to have to keep her group together through all of this adversity and hope that 2012 proves to be a better year for the group.
Like always, I have a 24-hour rule before I start spewing paragraphs of text on how I feel one way or the other, but still, it's really hard to take all of these graduations in such a short period of time.
EDIT: The official H!P website confirms it. Yuuka's gone after this year.
EDIT 2: TKMR has translated Yuuka's message. An excerpt:
Since becoming a high school student, I have been unable to keep up with my Smileage work and school at the same time and it has bothered me a great deal. I started to think about my future and realized that my desire to attend a university had grown a lot over time.