The service area for which a subdivision or residential development shall be considered to have impacted, shall be the entire Snohomish School District. The District encompasses a geographic area in excess of that of the City of Snohomish; therefore, impact fees cannot be directly attributable to a specific geographic area at all times. This is particularly true for junior and senior high schools. The School District shall, however, attempt to designate impact mitigation for elementary schools, as much as possible, to the general geographic area in which the subdivision or residential development is located, especially in such cases where the school population for the subdivision or residential development is within what is considered normal walking distances between home and an elementary school or school site.