trouble in the section

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ok so in marching band i got the position of section leader and alot of people were pretty ticked in the beginning cuz im a Junior... but i ended up working out all of those problems. Now, I am havning problems with the other Squad leader who is in the same grade I am. She laughed at our foreighn exchange student when he got confused and argues with me all of the time in front of the entire section which undermines me and makes everyone else think they dont hav to listen to me either. Im not sure what to do. I have tried talking to her best friend about how to talk to her and she ended up just making up excuses... when i went to talk to the squad leader, she also made excuses and said i was being intense... but no one else has a problem with what I have done so far (i ask their opinion most of the time to make sure its not over kill, etc). Any one have any ideas on how to deal with this?