Having delt with extreme bullying in Junior high and early high school I can understand how you might be feeling. Much like when you lose control of your car you must turn into the swerve by just agreeing with any insults. Do this until you find an opening to tear them down with a well placed insult. As to the threats of violence you must nip that in the bud. If he whispers a threat to you in class repeat him overly loudly. Hopefully a teacher will overhear and come over. If he threatens you away from a teacher again turn into the threat by stroking his ego "ya you could kick my ass you are fuckin buff!" If this type of tactic fails and a fight is coming remember to strike hard, fast and dirty. Eyes, balls, nose with quick puches or kicks. Having been jumpped by 2-3 guys weekly for 2 years I can tell you fights can't always be avoided, but you will live and be stronger for it.