A former Pelham High School assistant principal was arrested and charged with having sexual relations with a student.   33-year old Catherine Bell of Alabaster was placed on leave while the matter was investigated, and resigned last month, according to the Shelby County school system.   Bell was being held on $90,000 bond at the Shelby County Jail.    A statement from the Shelby County school system said Bell reisgned Nov. 21.