Advice & help, also Joy from someone else karma?

Its this girl , im normally the type a person that will curse somebody out if they say the wrong thing to me, but this year, I kind of semi-changed a little. There's this girl, lets call her Kate, so kate seems like she's trying to upscale her self and become RUDE while im on the other hand is trying to lower my "RUDENESS LEVEL". Kate rudeness kind of started a few weeks back, i was like " i want a moped, i think its kind of nice" then, kate said " im sorry but i cant imagine you on a moped because you kinda big and big people shouldnt be on mopeds" (That was not even caused for) while my other friend just listened and didnt say nothing, which is better, even if im living in a little fantasy. Now a couple days ago i was sitting next to kate, my hair was nice and alot of people told me so, and kate just stared at me, so i got up and was doing some work in cl then kate said "you know your is different, you know . . . im not tryna be mean, no offence but you know like them real big people and their is like different , no then she draws this picture to show me how my supposably looks and its basically showing like my is flat and saggy" then later in cl this boy started talking about kate, kate is way smaller than me but she's not skinny and really greedy so this boy started calling her fatty and saying you fat , and then calling her a pig, and just putting her down on her weight, and as i sat their i realized wow, she's getting her " medicine " back from all the bull **** she tried to say to me so my question is " What do you think is wrong with kate? is she jealous like whats wrong with her? and also, i realized i had kate notebook and our notebooks at school are filled with a lot of work in it that was due and i realized most likely kate wont be able to study or her work most likely wont get credit because we cant turn in late work, so is it okay for me to feel a little happy, since kate tried putting me down and now she suffering ?