Actually, I'd go all-out in the opening:
"May it please the court:
This case...is...fake.
The witnesses you're going to hear from today are not real people, the crimes you're going to hear about never happened, and the facts stipulated to by both sides are entirely fictional.
You see, judge, you are 'presiding' today over a 'MOCK trial.' This means that, unlike typical sentencing hearings, the people arguing before you are not licensed practitioners before the bar, and the defendant has not actually been charged with a crime, because the defendant does not exist. In fact, this entire proceeding is an elaborate hoax, without any grounding in actual jurisprudence.
Your honor, none of this is real. None of it. And that's why, at the end of this mock trial, my co-counsel will come before you and remind you that the stakes couldn't be lower, the facts couldn't be more fictional, and the outcome of your decision is completely irrelevant. Thank you."