I NEED FREAKING HELP!!?

I have been getting weird sexual letters from this guy ok short summary he was being a jerk I got mad at him for it, he started stalking me and I told the teacher but he told him to write a letter of apology and inside the letter was a condom and he started writing me sexual letter no joke I got one write now, it says stuff im uncomfortable saying and I'm getting really paranoid!!! I havent eaten or slept in days and my parents say I need proof it can be anybody since their is alot of horrible people in my school, I think I'm really close from my breaking point.... Help this guy is Goliath he is huge I don't want to get beaten up.. Or raped.