Immaturity in college

Okay, so, I'm in college and resitting English and Maths because I dropped out of highschool in year 10 (age 15) due to mental health. Tell me why there is still girls riding the high of being popular in high school..

So I walk into English, and as I always do I make my way to a seat at the back, and I pull out a chair, keep in mind that my hand is ON the back of the chair. I am holding the chair!! And then, this girl walks over and just sits in the chair, with my hand still on it??? So, I ask politely "Sorry, did you want to sit here?" and I step back, and she literally gives me the dirtiest look possible and says in such a bitchy voice "Well I'm already sat here." BUT YOU COULD SEE I WAS GONNA SIT THERE?? But oh well, so I move to the table next to that one and sit down, without realising I'm surrounded. I have that girl and her gay bestfriend on my right, a popular girl on my left, and then two more popular girls in front of me. They spend the whole lesson mocking me and giggling as if I'm not right there.

We are 16/17 years old.. and we are still immature enough to mock others appearance? Please grow the fuck up, and just do your work, because why would you waste time mocking me for simply existing when you could do your work and get out this class quicker so you can focus on the actual reason you came to college? I can't comprehend the thought process of these girls. Please tell me I'm not the only one dealing with such immaturity