Persons employed to teach in and administer the program improvement schools shall be deemed to be on leave of absence from the school district whose employment they leave to accept employment in the program improvement school, and shall retain permanent status, or credit for service toward permanent status, in such school district during their employment with the program improvement schools. Such persons shall continue to be members of, and contribute to, the retirement system of which they were members at the time immediately prior to their employment in the program improvement schools.