(a) the repetition is expressly authorized by another provision of this article; or

(b) the student files a petition and the governing board of the district or its designee grants written approval of the petition based on a finding that the student's previous grade (whether substandard or passing) was, at least in part, the result of extenuating circumstances. Extenuating circumstances are verified cases of accidents, illness, or other circumstances beyond the control of the student. When course repetition is approved pursuant to this subdivision, the district policy may allow the previous grade and credit to be disregarded in computing the student's GPA each time the course is repeated.