Maybe I've read this wrong and she's just got some issues.
However, from what you've said, Shes amusing herself. She's a bully. I'm guessing she's got a follower or two to observe her antics and she's dying for a confrontation.
I think that she's got an expectation of seeing you so she can carry on her antics and as a first try I think it would be better to change your timings slightly so that you can avoid her and break the cycle so she tires of waiting and finds some other way to entertain herself.
Out of sight out of mind might be enough to shake her off. I'd try that first before challenging her. It may sound a bit cowardly, but its a first step.
The other thing is safety in numbers. Can you find some other mums to walk with? She is less likely to try this if you are in a group. They would also be good witnesses.
You could also try grey rock. She's looking for a reaction, so don't give it, as others have said, Sunglasses, iphone earbuds, conversation with other mums... act as if she is invisible.
Have a word with the school too, and let them know. You might not be the only one. if you feel they are approachable.