Children have no tools, and are forced into these situations with no possibility of escape. The thicker skin you describe doesn't exist for the majority of the planet.
My parents tried all they could to shelter me behind regulations, and even they -- a colonel and an elementary school teacher -- failed and eventually admitted I have to stand up for myself if everything else fails. I stood up to (and beaten the crap out of) bullies and even stood up to teachers if I had to.
The people who don't care about the rules are the people against which bullied and harassed teenagers have no one to count on other than themselves.
My parents didn't teach me to run to them or to teachers when stuff like this happens. They taught me how to deal with it myself.
Treating high schoolers like they're helpless kids might be fashionable now, but it might not seam as wise when they're 30 and living at home.
Most everyone has had this problem at one point in time. It seems to me that mom needs to teach her daughter to have a thicker skin...which is the EXACT same lesson I got from my parents.