Frustrating

Today I was called in to supply for a grade 8 class for the afternoon.  When I arrived the teacher was still there and we talked for a few minutes about what my afternoon would be like.  She started off by telling me I would be rotating between her class and the grade 7 class for math.  I was really excited by this, as it would give me more experience for this class.  She started going through the lesson she was going to teach and from what I saw it was going to be really good.  Then she said its a little complicated so you can just watch a movie for both classes.  I told her I was capable to teach the lesson and that I was upgrading to my intermediate in math.  However that did not persuade her and she closed up her notes, handed me the movie and left.  I am really sick and tired of teachers thinking just because I am a supply teacher that I don't know what I'm doing.  Sorry for ranting, just wanted to blow off a little steam.