I've heard of the term Big Hairy Audacious Goal (BHAG) before. In fact, I have previously worked at a place where BHAG's were commonly put forth by employees (it was a management company for a franchise of KFC's, their goals were clearly and constantly stated, truly one of the best companies I have ever worked for).
At another previous employer, we discussed having a creed, a written description of our beliefs and values. Writing this creed was an eye-opening and, ultimately, rewarding experience. I was forced to think about who I was, what I expected from myself, where I was going in life, and why it all mattered (on a side note, it was that job that largely encouraged me to go back to school). BHAG's and creeds inhabit the same space for me, in the sense that they are written descriptions of who we are and what we stand for. They are, and should be, ambitious, forcing you to think outside of your current capabilities and inhabit a new space where you are challenged. What better place to encourage ambitious, innovative, and mentally stimulating goals than at a school library?
Our jobs as school librarians, in my mind, is to help students break outside of their normal box of thinking and doing, and inhabit the "what-if" spaces. Exploring and daring the world around them to challenge their thinking and perceptions. Granted, this won't necessarily succeed with every student, every school, in every situation, but if our mission statement, clearly stated, helps others (i.e., teachers, administrators, parents, etc.) to step outside their comfort zones and try something new, then we've succeeded.
Some would argue that mission statements are tripe, put forth by administrators and executives who have too much time on their hands (which is true in some cases). School libraries should have mission statements that align with the school to clearly state who they are, why they are important, and what we should expect from them in the long run. We expect students to be define their long-term goals, why should we expect any less from ourselves?
If you're wondering why I don't share my creed (which is me really assuming a lot), it's because I've saved it somewhere on my computer and can't find it. I need some sort of cataloging schema for my files.