6 year old and problems at school

My 6 yr. old daughter was pretty normal as far as behavior problems until this school year. She is now disruptive and getting into fights. She also gave a kindergardener the middle finger. None of this behavior is learned at home. She does have a 2 yr  old sister of whom she is very jealous. Most of her bad behavior is at school. She does lie on occassion at home. Time out, grounding, and other similar discipline techniques are not working. I need some advice. My husband thinks we are not strict enough and my mother thinks I am too strict. I think we do exactly what we should, but obviously it is not working! Please help!

[Sandman2]

  The problem is - at this age, there is not a lot that you can do at home to punish for behavior at school.  The kids developmentally wise just can't connect the two.  In other words, anything you do at home to punish for behavior at school is a waste of time.  Probably by about the age of 9, home punishment may work.  What you can do at home is to model or teach behaviors that will help her at school.
  Essentially, the school has to deal with the situation.  Kids at this age need immediate reinforcement.  To try and do it hours later is nuts.  So this is really something that the school has to deal with (and I do have some experience in this area).  You do need to find out what is causing this behavior at school, because something is going on.  It may be who she is hanging out with.  It may be the teacher or playground people have no control.  I kind of wonder what the experience level of her teacher is.  To report home that she flipped off another kid, speaks more of a frustration level act by the teacher.  So essentially, what you are doing may be correct - although good modeling of expected behavior at home does help - its the school that needs to step it up.  And do realize that nothing happens overnight.