Find the Treasure - Read Your Agreement

Historically, unions came about as workers banded together to achieve a common goal: an improvement in working conditions and salary.  Like-minded members selected leaders and through them, their union bargained with the employer on behalf of all members.  This included the negotiation of wages and benefits, the rules governing hiring, firing and promotion of workers, the process for resolving differences, matters of workplace safety etc.  As they were at the onset of unionism, these  negotiated agreements are binding on all members and on the employer.
Let me highlight here, a few provisions  that you should know. Note that these are found within the first 4 pages of your Collective Agreement - there are 33 pages in total:

• The Board or school/work site Administrator shall notify the member of his or her right to Union representation.  The Union shall be present at any meeting that may result in the discipline of a member. 
• A Short-Term or Long-Term Occasional Teacher hired to replace an absent Teacher, properly subject qualified for such position, will not be replaced by another Occasional Teacher other than mutual agreement of the Occasional Teacher and the Board.
• An Occasional Teacher must be a member in good standing of the Ontario College of Teachers to be eligible for inclusion on the OT List.
• An Occasional Teacher shall notify the Human Resource Services Department in writing, of any changes to name, address and/or phone number ...
• For the purpose of probation only, a half day equals one full day recognizing that the maximum credit shall be one full day.

Remember that the onus is on you to know your Agreement.  Ignorance of the terms is not an excuse backed by arbitration.