In schools without vice-principals and when an assistant to the principal has been appointed, such assistant to the principal shall be paid in addition to salary according to scale, Article 5.09, an allowance of $2058. Effective September 2015, the allowance shall increase to $2100. Effective September 2016, the allowance shall increase to $2142. Effective September 2017, the allowance shall increase to $2174. Effective January 2018, the allowance shall increase to $2206.