Concentrate on simply instilling within your daughter the characteristics of integrity, kindness, compassion, the willingness to work hard, and other values that are universally appreciated and needed in this world of ours.
There will always be a "Mary". Right now Mary is a 12 year old girl. Eventually, the "Mary" will be a teen girl, a jealous friend, a truly evil person, a misguided but well-intentioned person, a person who made an innocent mistake, a roommate, a boyfriend's ex, a boss, a co-worker, an in-law, the list goes on and on. Some people have a lot of Marys in their lives, some only have a couple. But everybody has at least one.
If you try to fix everything, you'll go crazy, and your daughter will have no ability to advocate for herself, your daughter will not develop maturity, and no matter how many Marys you deal with, there will be another one.
So focus inward. Don't go to the teachers, the parents, the corporations, or anything else. Strengthen your daughter. Don't ask things like "are you going to let this ruin your life?" Don't use other people's bad behaviors as examples of what not to do. Instead, just teach your daughter what she values, what is important, and what to stay true to. Give her the tools to handle what life brings.