As many on the forum know, I attended one of the NE boarding schools often discussed here. There, and at many BS, up to 30% of the class are repeats. The college counseling office was clear - you report all schools attended from 9th grade on, regardless if any credit is awarded from the old school
And the questions on the common app are not ambiguous: “If you have attended any secondary/high schools not listed in the previous section, please indicate the number of schools.”
Equally unambiguous is the affirmation section: "I certify that all information submitted in the admission process — including this application and any other supporting materials — is my own work, factually true, and honestly presented…’