I don't want to fill in for my friend for a prom assembly
Why don't you want to fill in for their prom?
It's too much work for me. I'm not his slave
It can be a lot of work to do that. But be a good friend and help out, don't back out on your commitments.
He's lucky for having me. I'm such a good person. Just kidding, I'm not. I'd do anything for the best cause.
Try to follow through on your promises unless you absolutely cannot. Your friend is depending on you and I am sure you will depend on them in the future. Helping them out will strengthen your bond.