A school district may purchase, acquire, own, manage, and hold title to real estate for future school sites which at the time of such purchasing or acquiring is outside such school district in a territory not more than three miles beyond the limits of such district but contiguous thereto. Such district shall not erect school buildings on the real estate prior to the inclusion of such real estate within the boundaries of such a school district. If the real estate so acquired adjoins the purchaser’s district, the acquisition of the real estate constitutes an annexation of such real estate to the purchaser’s district. The intervention of a street, road, or highway between the real estate to be acquired and the purchaser’s district does not preclude such real estate from being considered as adjoining the purchaser’s district.