A teacher, being a member of the Manitoba Teachers' Society Executive Committee or of the Executive Committee of any branch thereof or any special committee of the Society, or being appointed an official representative or delegate of the Society, or any branch thereof, or being a Society appointee to a committee of the Department of Education, and being authorized by the Executive Committee of the Society to attend a meeting of the Committee of which they are a member or to act as a representative or delegate of the Society or any branch of the Society in a matter of Society business requiring absence from school shall have the right to attend such meeting or to act as such representative or delegate and shall be excused from school duties for such purposes on not more than a total of twelve (12) teaching days in any school year, provided that a substitute satisfactory to the Board can be secured and that the cost of providing said substitute is assumed by the Society and shall not be charged upon the Board concerned. No additional leave of absence shall be taken for the purpose mentioned above, except with the consent and approval of the Board.