Private insurance will often pay for a limited number of speech classes or at the very least, an evaluation by a professional speech therapist. Ask your pediatrician to refer you to a speech pathologist. Have your child evaluated to see if the therapist concurs with the school's evaluation. Private speech therapy classes are prohibitively expensive (I was quoted $90 for a half hour), but the therapist should be able to offer suggestions for working with your child. If there is a serious problem, your insurance (or even the school) should provide the help she needs.
My daughter had a lisp, as well as pronunciation difficulties. At eight years old, the improvement has been dramatic. We did not pursue outside help. Although most books on speech development mention seven as the cutoff age, a friend who specializes in speech difficulties told me that eight or even nine may be more accurate. That has certainly been the case with my daughter. Good luck!