In cities of the first class the president of the board of public education be and is hereby authorized to deputize the vice-president or the assistant secretary of the board to sign any or all warrants legally drawn in payment of salaries of teachers, janitors and all other employes in the public schools after the approval of the pay-rolls by the said board: Provided, That such authorization shall be in writing filed with the city controller, and that all of the said warrants shall be countersigned as heretofore by the secretary of the board: And provided further, That nothing in this act shall relieve the said president from the duty of signing all other warrants of the said board.