Section 9D. No person permanently employed as a teacher in any institution under the department of mental health, mental retardation, public health, public welfare, correction, or youth services, who is not classified under chapter thirty-one, shall, after having served as such teacher for a period of three years in such institution, be discharged, removed, suspended, laid off, transferred without his consent, lowered in rank or compensation, nor shall his office or position be abolished, except for just cause and in the manner provided by sections forty-one to forty-five, inclusive, of said chapter thirty-one.