If, in any year, the number of children entitled to receive benefits in accordance with N.H. Rev. Stat. § 198:24 shall exceed the amount appropriated for such purpose, the amount shall be prorated proportionally among the districts entitled to the tuition payments. In carrying out the proration, all sums appropriated to the foster children tuition fund established by 1975, 505:1.06, 03, 03, 01, 21 which have not been expended or encumbered on August 30, 1977, shall be divided equally between foster children placed in a program or school for persons with disabilities and foster children placed in a group home or nonprofit institution which averages 6 or more foster children annually. After this division between foster home groups and disabled foster children programs, said sums shall be disbursed on a pro rata basis for each disabled foster child up to 100 percent of actual costs and on a pro rata basis for each foster child in group foster homes. In subsequent years, this same pro rata distribution shall be made of these foster children tuition funds.