For the 1990-1991 school year, the Commonwealth shall pay to each school district, which has an actual instruction expenditure per weighted average daily membership for the school year prior to the reimbursable year, which is less than three thousand four hundred forty-five dollars ($3,445), and has ten percent (10%) or more of its pupils in average daily membership as children in low-income families, an amount equal to one-half percent (.5%) of their actual instruction expenditure.