A rough few weeks!

Emily has been getting into a lot of trouble at school lately.  Rob and I have absolutely no idea what to do because, honestly, she is not a terror at home.  We have been receiving telephone calls during the day from the assistant director at her school and he has been asking us to talk to her about her behavior.  It is so stressful because every time we see their telephone number on the caller ID, we assume the worst, like something has happened to her, but instead, it's just our daily discipline call.  Neither Rob nor I ever act like Emily is a perfect child; afterall, she is a little person and I have yet to meet anyone who is perfect.  From what the teachers tell us, she is extremely disruptive and disrespectful which is disappointing to us.  I just feel like my hands are tied.  I have no idea what they expect us to do when we are not there to correct her bad behavior.  She is 3.5 years old...most of the time, she doesn't even realize what she has done wrong by the time we talk with her.  It's incredibly frustrating because we really like the school but we are beginning to question thier discipline practices.  I feel like I am constantly being beat up...I'm always worried about what people think about me and I don't want anyone to ever think that I am a bad parent.  I love Emily more than anything and I am constantly trying to teach her to "the rules."  Rob and I are exhausted and overwhelmed and since I am always so optimistic, all I can think about is how are things going to be when she starts "real" school or how will she behave when she is a teenager?  I know that I am overreacting but we just don't know what to do.