The convent scenes were filmed at Ramona Convent Secondary School in Alhambra, CA, an all-girls Catholic Jr. High and High School. The school's main building, where most interior and exterior filming was done, was irreparably damaged during the Whittier Narrows Earthquake on October 1, 1987, ten days before this was aired. The main building was demolished on December 11, 1987, 18 months short of its 100-year anniversary.
Although the building used as the convent dated back to 1889, it did not have any secret passages, underground or otherwise.
Brennan and Gulager also costarred in the 1971 movie "The Last Picture Show."