Section 38D. Two or more towns may, by vote of each, form a district for the purpose of employing a director of occupational guidance and placement, who shall annually be appointed by a joint committee composed of the chairman and secretary of the school committee of each of said towns. The committee shall determine the relative amount of service to be performed by him in each town, fix his salary, apportion the amount thereof to be paid by each town, and certify the same to each town treasurer. Any constituent town may, by vote passed prior to December first in any year, withdraw from the district, with the approval of the department, such withdrawal to become effective on June thirtieth following. No city may join in forming such a district.