Peer Pressure in 1st Grade?

I have a 7 year old daughter that is about to head into the 2nd grade.  She has always been a well behaved, respectful child.   Recently, my husband and I notice that her behavior is changing.  She is displaying more of an angry, disobedient streak. Things are going downhill and I don’t know what to do.  Today I asked her what was wrong and why she was being defiant.  She told me that she does not have a lot of friends in school.  No one wants to hang with her because they said she is not cool.  In reality, she does have friends, though they are few and have an innocence to them as my daughter does.  What I’ve noticed is that the children that are making these statements do act more mature than she does; some are into bullying, hitting, swearing, etc…   My question is should I be worried about changes in her behavior, is peer pressure starting in 1st grade now, and how do I address these type of issues?  Do I hope this will fly by and things will get back to normal during summer break?  Nowadays, children are growing up to fast, I want to my child to be just that, a child, as long as possible but I am afraid that peer pressure is going to 1) make her change(which we are starting to see), 2) cause her to become withdrawn, or 3) get completely out of control.   Any thoughts???