Sorry Oli, I'm going to have to agree with the majority. You sir, are a moron for not going STRAIGHT to a lawyer and suing their asses off.
They jepordized your future just so one worthless guidance counselor could save her own skin. They can say the GED is the same as a diploma all they want, it just isnt though. At least not in the eyes of those who make the admissions.
Regardless if you signed something that you agreed to the "Zero Tolerace" rules, that still doesn't give them a right to expel you for something you had no control over. You have a copy of your schedule right? Does it show said class you were supoosed to go to? If not theres your proof.
The bruden of proof in court would lie in them and not you. Any lawyer worth their salt could nail that school to the wall.
For some reason I feel something is being left out of your side of the story.