A.S.
When I was 12 I walked to and from school. It was about 1.7 miles. I never crossed any road over two lanes each way and my neighbor hood was right next to the administration building for the school district. It was very, very safe. I understand that it has been 20 years since then (I'm aging myself here) and things have changed but it really depends on the route she would be taken. In my honest opinion, if she doesn't have to cross any major streets I would probably be okay with it. As long as she knows safe practices when it comes to walking and strangers/predators I would think a 12 year old would be perfectly capable of safely getting to and from school; especially if she has an older sibling walking with her. It definitely depends on where you are living and what types of neighborhoods she would be walking through.
A couple of suggestions. While school is out walk the route with her. Show her which routes would be safest. Time how long it would take her to walk the distance. Have a very long discussion between the two of them about safe walking practices. See if there are any friends that will be walking the same route (all the way or partial) and talk to their parents. She is going to have to learn how to do this at some point. Just teach her how to do it correctly and safely.
ETA: Buy her a cell phone; they have ones you can track now as well. Make a rule that you call as soon as you get to school and when you get home. This is what my bosses children do and they are 12 this school year.