How would you distance yourself from someone? 10pts?

I am in charge of fundraising at my son's school and know a lot of parent's at the school but all of my "friends" live out of town. Well this year I have become friends with another mother and we have both invested a lot of time trying to build the friendship. I am wanting to distance myself from her because she is extremely negative and continuously thrives on conflict and strife. She always has a problem with something or someone and she has unrealistic expectations from everyone else around her. I know that it is just a matter of time before she will have some kind of problem with me and I could never live up to her unrealistic expectations. I am human and I will inevitably make a mistake that she will have a major melt down over. I would like to distance myself from her as much as I can but I have to see her at school everytime we have a school function. Does anyone have any thoughts or ideas that they could share with me? Thank you.........