My sister can't run in P.E. Help?

My little sister just started 7th grade. They expect her to run the mile in 9-11 minutes. My sister is a chubby girl, and she can barely run. She couldn't run the mile, so she walked. The PE teacher told her that she has to come to her during 0 period to run, until she can finish a mile in less than 11 minutes. She is really scared, because she can't run at all, and the teacher is pestering her. What can she or my parents do do resolve this problem besides making silly excuses?