There isn’t a universal photography definition for “the conventions and dignity of standard portrait photography.” In this context, it’s essentially a local yearbook policy, and the school staff or teacher in charge usually gets to interpret it.

Technically, your photo may satisfy the listed specs, but publishers—including schools producing a yearbook—can still reject images they feel don’t fit the publication’s style or standards. A landfill setting may be seen by them as conflicting with the “dignity” or intended tone of a traditional senior portrait, even if you disagree.

So this is less a photography-rule question and more a policy/editorial-decision issue. If you want to challenge it, your practical options are to ask for a clearer explanation, request a review by the yearbook adviser/principal, or submit a different image that fits their expectations more obviously.