Why are people stupid?

So today has not been a good day. I am just going to say that. I tried so hard to look at the positive side of things today but sometimes I just can't. The first thing that made my day horrible was that I did not do very well on my Bio test. But, during the next period (science) I forgot about that but we got a booklet of worksheets which take forever to do. Then during my spare my day went from bad to awkward when this guy said that me and this other guy should have sex.(It took all I had to say "Ew no." cause he was there as well). Lunch was okay I guess. English was good. I sat beside my friend Taylor and the half period went by really quickly. Math was stupid as usual (My teacher got so confused/frustrated he just let us leave but it was only a couple of minutes early). After school was the worst though.I was waiting for my bus which was really late and then my sister came over and starting yelling at me. Basically, it started off with her saying to know the whole story before I say anything cause I was making fun of her for "getting lost" when she did it on purpose. Then she continued to say I was a "F***ing c***" and that I had no friends. Then she turned to the girl I was standing with and said that she shouldn't be friends with me because I would just stab her in the back. And then she kept calling me a horrible person. I then said that I really did have friends but then she said that no one really liked me and everyone would be better off without me. I know I shouldn't care what people think of me but when that comes from your sister it really hurts. It took everything I had to not start crying right there. But i didn't cry. I walked away and waited for the bus inside. Then the bus didn't even come until like 2:45 - 2:50 ish. When I got home my day got somewhat better we had a really good dinner but my sister still swore at me when my mum wasn't home but now that she is she is pretending that nothing happened and that we are best friends. And I had to stay home from dance cause the roads are to slippery. So yeah all in all it was a horrible day. Sorry for the depressing post. I just needed to vent.