Instead of fighting try to defend yourself. There is a difference, despite what a lot stupid and ignorant people out there say; who like to not only blame people who do hurt others, but also to those who are simply trying to protect themselves.
The only way you get out of this is if he's the one who is seen causing all the trouble by making him get overzealous and stupid. Cause if you and the dude start wailing/wrestling each other (like a lot of fights end up in), then you're not going anywhere with the situation and you'll both be blamed for the fight.
Like I said defending yourself is a better solution than trying to beat him up. Calling the cops (like others suggested) is a better solution, but with the way today's schools are teaching these kids (and its a fricken crime IMO) they have to understand that there's not always an adult (or in this case a cop) around and defending yourself would be your best option.
In the end if it does goes down let him throw the first punch (while defending yourself of course) this way you can have something for the cop so that he can make an arrest afterwards.
He's threatening you; might as well take advantage of it.
