N.H. Rev. Stat. Ann. § 195-A:13

Whenever several school districts have adopted and established an authorized regional enrollment area plan as provided in this chapter but have not applied such plan to all grades of elementary and secondary schools in the area, they may subsequently extend such plan to all or part of the omitted grades by establishing a new area school planning board as provided in RSA 195-A:3 and by proceeding as provided in said section to prepare a supplemental authorized regional enrollment area plan for such additional grades.