My Asperger's daughter has scalp sensitivity and went through a phase in Jr. High where she would "shampoo" her head by squirting shampoo in her hand, barely touch her head and rinse it off still in the gel form. I thought she was thoroughly washing because her head was wet out of the shower (and she didn't smell) but later, when her hair dried it showed greasy strands.
Reminding her to do a better job went on for weeks. We had her hair cut short, made her go back and shampoo again when we caught it, and finally I stood in the bathroom and watched her shampoo thoroughly (she was mortified). She got better about head hygiene.
It took a concerted effort though. No amount of peer pressure made a difference, as she is semi-impervious. Is it possible this girl has a disrupted home or preoccupied parents? Does she have autistic tendencies? If the other kids are cleaner along with the parent, this girl might have some individual issues that need attention. Talking to her parents is helpful. Maybe if more people speak up they'll step up the hygiene lessons, but remember, it's not always neglect that causes problems.
*I would speak up though because it is affecting others.