I remember reading during the first season that once the character's graduated they were done, as it was a high school drama, so it's been out there for over a year and shouldn't be a huge surprise to anyone and actually most of the characters will be seniors next year and thus gone, which should be interesting to see how they handle it. I think it's neat that they're keeping the focus on the school/glee club, not on the specific characters, like so many shows do.
I agree that you could totally keep Finn around by having him fail to graduate, doesn't work for the rest of the cast though.
I think the producer probably should have been clear with the cast, but the news has been out there for a while, pretty much from the beginning, so they can't really have been blind-sided by it.