Running ability is a legitimate criterion for grading students in a gym class. In general, there are several guidelines teachers must follow for grading procedures.
First, identical criteria must be applied to all students in the class, except in those cases where the teacher is accomodating documented disabilities.
Next, the goals of the class should be made clear to the students so that they have adequate time to prepare for assignments or exams.
The teacher must follow accomodation plans for documented disabilities. Thus, for example, students in wheelchairs could not be required to run 2 miles, but might be asked to perform other equivalent activities.