that fricken sucks, I wouldn't go, I'd also explain to the teacher

That might get him in even more trouble... best to just give the teacher a live handwriting demonstration or take the issue to an administrator.

Show them that's how you write now. That should solve it. If not, just Breakfast Club it and hang out with the people in detention.

Should have been pretty simple to prove that it's you're handwriting before it got to detention stage, unless they didn't even give you a chance to defend yourself.
Just write something out!

(Whatever happened to edit here?)

Anyway, I wanted to add that I also wonder what this teacher will do when all your future assignments start showing up in that same handwriting and they realise that they falsely accused you of cheating?

And that is why I always typed and spell checked my assignments.

Some teachers look for any excuse to give out punishments. One teacher I had tried to give me half a day of in school suspension because she couldn't read my handwriting OR understand the shorthand I was using to take notes. As in, notes purely for my own study. She made me actually read it to her and explain it.