A provisional member is a person who is employed by the board, the department, the New Mexico school for the deaf, the northern New Mexico state school, the New Mexico school for the blind and visually impaired, the girls’ welfare home, the New Mexico boys’ school or the Los Lunas medical center and who has the option of qualifying for coverage under either the Educational Retirement Act or the public employees retirement association. This option shall be exercised by filing a written election with both the director and the executive secretary of the public employees retirement association. This election shall be made within six months after employment and shall be irrevocable regardless of subsequent employment or reemployment in any administrative unit enumerated in this section. Until this election is made, the provisional member shall be covered and shall be required to make contributions under the Educational Retirement Act.