How do I make a roast speech on my classmates if I am not even funny?

I told my speech teacher that I want to make a roast speech to everyone in the class, but I recently found out that roast speeches is supposed to a tribute to someone with humorous intention. I have made people laugh with my comments and facial expressions, but I am not funny because I AM NOT! My speech teacher, he told me that I cannot change my options for the final speech.
How do I make a roast speech on my classmates if I am not even funny?
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