My daughter, who was almost 6 when her brother was born, never expressed it in the ways that you describe, but she was definitely experiencing similar feelings.  There was some unpredictable acting-out behavior etc.  I think she never really got used to her brother because she had been the "only" for so long. We made a special effort to plan to do things just with her alone (and usually just one parent).  She also spent one-on-one time with each of her grandmothers.

That kind of behavior really suggests low-self esteem. I always wonder if there is something going on at school that you don't know about (that was true for us) -- bullying or some kind of peer pressure? Does she have a friend or 2? As first steps, I would talk to the teacher.  But also, have you spoken with your pediatrician?  This could just be transitional, but since the behavior is so specific (she doesn't like her face) maybe your doctor could help you understand more about it and point you in the direction of some specific help. 

If you go here you'll see the list of information we have available on Parenthood about sibling rivalry (although most of it deals with preschoolers and toddlers you may find some general advice...)

Elizabeth