What does the scene of Helen fighting with Martha and Percy teach us about social classes?
To be honest, I'm less sure it shows us specific things about class and more a blend of things about race and class. By that I mean, it matters a lot that both Martha and Percy are Black. This is set in the South, and Helen's parents dwell on the Civil War; we can assume some residual negative attitudes towards race.
That said, Helen is allowed to play wildly with these two and they with her. She's treated more as a thing than a person—they get her so mad she bites her own fingers, after all. I'd say that it is as if her limited senses allow her parents to demote her in social class.
The scene of Helen fighting with Martha and Percy tells us no matter what social standing you are in in the social hierarchy, you may not behave in an refined or dignified manner, but instead show an unruly and not lady-like behavior. They are allowed to play or fight with each other like barbarians and they even behave like monsters, even doing despicable and gross things- like the aboriginal people, bitting their own fingers when they played too wild. It shows that no matter what classes or what social ranking you are in, your behavior might not be what people highly expecting you to do.