Social events and Optimus Prime are like oil and vinegar. Often side by side but usually never mixing, and if they do, it's trying its hardest to separate again. But Shockwave had convinced him to come to this get together. And by not even entering the competition he'd somehow won Prom King. Yeah...you figure that one out. In celebration, there's a small crown that didn't quite fit on his head, so it's looped around one of his giant blue ears for the evening.

This was going to be a long night.

Option 1:

You can find him chaperoning against the wall, making sure that everything is going at least somewhat peacefully--people are having fun, dances are being had. Things like that. Optimus dancing, though? That's asking far too much from the Autobot leader. Not only does he openly claim he has two left feet...

But maybe he just doesn't know how.

Probably a bit of both.

Option 2:

He's standing outside to get some fresh air (you don't breathe, dingus) from all the bustle.

What a party animal.