I'm kinda freaked out....plz answer me?

There is this guy at my school and he said that I was a $lut.. which made me feel weird and sad. I have never acted like one, and my friends say i am very modest. The next day, he said he was sexually attracted to me and that he had sex with my sister. My sister is two years older than me, and she has never even dated someone. He has been giving me weird smirks in the middle of class, and I try to ignore him. But, every time he thinks I am looking at him, he points to his penis. I know this probably sounds gross, weird, and fake.. but WHY would I make this up.. thank you for listening