I believe the statement is based on human nature. You honestly don't think people (even teachers) make assumptions/judgments based on appearances?
This is absolutely true.
My husband is a striking example of how you are treated/what others perceive based on appearance. He was the only child of a poor single mom, long-hair, stoner-looking teen. Absolutely nothing was ever expected of him in school-- he was constantly told "college isn't for everyone", he was placed in classes for kids with learning disabilities, guidance counselor told him he should just drop out since he would never get a job that required a diploma, the list goes on.
He did wind up going to college (had to take pre-requisite classes at community college first because he wasn't offered the proper classes in high school) and has had a successful nursing career. But he maintained the same appearance for at least the first decade of our marriage. (The mindset was that people shouldn't judge based on what you look like.) He was profiled by police, people were rude to him in public (always fun buying cars or furniture when the sales people don't want to assist you or are flat out disrespectful), even at work if he was outside of his "area" people who didn't know him assumed he was a custodian, talked down to him, etc, etc.
He one day decided to get a very conservative haircut and buy a new wardrobe. The change in perception has been almost unbelievably drastic. I knew that people judged based on appearances, but I honestly never realized the full scope of it until now. Within the last five years (since changing his appearance) he has received many job offers and been promoted several times (he's recognized as an expert in his field and oversees multiple hospitals), he is a well respected member of the community, and even his own family's perception of him has changed.
Do I think it's fair or right to treat people differently based on their appearance? Of course not. (I know DH didn't magically change into a different person who was somehow more deserving of respect when he cut his hair and put on new clothing.) I don't have to agree with it, but I do have to recognize that's what most people do.