REALLY good excuse for getting out of cross-country?

Okay, I'm not in a cross country team, and I go to an intermediate school, and at our school cross country is a once a year thing that everyone has to do. Please don't tell me to just do it. My parents wouldn't write me a note, and how the hell am I supposed to get one from a doctor? It's tomorrow and it's half 7 at night here, I'm going out with the dog, and my mums coming and shes expecting me to jog round two fields while she walks, I'm thinking maybe I could trip myself up and graze my knee (not really badly, like so much that I need to see a doctor) but can someone tell me how? Is there a way of getting myself to throw up or something? PLeaes help, I'm worst in the school at running and I always come last, and it's REALLY embarring!!