When it is determined by the Board that a lay off is necessary and where natural attrition, transfers, sabbaticals and leaves of absence do not effect the necessary reduction in staff, a seniority list shall be developed as hereinafter provided. Such a list shall contain the names of those teachers having the least seniority identified in sufficient numbers to enable the Board to lay off the required number of teachers. The Association and the teacher shall be permitted a period of fifteen (15) teaching days after posting of such list to protest in writing to the Board any alleged omission or incorrect listing, but such protest shall be confined to errors in or changes occurring since the posting of the last seniority list.