Upon commencement of the sixth (6th) day of extended substitute teaching, a substitute teacher shall be paid for each day taught at the per diem rate (to be calculated as 1/x of the salary to which a teacher of the same qualifications and experience would be entitled under the basic salary schedule of the current collective agreement, where x = the number of days in the current school year). Such pay shall be retroactive to the first day of the assignment, and the rate of pay for the extended teaching assignment shall continue in effect until the end of the extended teaching assignment.