Section 15. The commissioners of the Kensington district of the Northern Liberties, in the county of Philadelphia, at the next annual election in one thousand eight hundred and fifty-one for school directors of the tenth section of the first school district, shall elect twelve school directors in addition to the number now authorized by law, so that the board of school directors for said tenth section shall consist of twenty-four members, and of the additional directors to be chosen as aforesaid four shall serve three years, four shall serve two years, and four shall serve one year, and annually thereafter said commissioners shall elect eight school school directors who shall serve for the term of three years.