I would suggest that you don't reveal your status in class, and if asked about it, to lie. You can tell your plan to your teacher beforehand. I agree, there are some real bad people in high school who will try to hurt you, in addition to the bastions of support like good friends and teachers like yours.

When prepping for the argument, take the role of someone in the third person (namely, a USC who supports DREAM and CIR in general) so as to avoid any emotional issues. Try to mix logic--what supports your argument--with emotion--what gets people to "turn on" to it and be more receptive to its logic. In fact, MLK was a master at this. If you haven't read his letter from Birmingham jail yet, do so.

Good luck and don't sweat it!