It shall be the duty of the assessor of each county, a part of which is included within a joint school district, to certify annually to the auditor of the assessor's county and to the auditor of the county to which the joint district belongs, for the board of county commissioners thereof, the aggregate assessed valuation of all taxable property in the assessor's county situated in such joint school district, as the same appears from the last assessment roll of the assessor's county.