This is as easy as setting your phone alarm to go off in the middle of the former class president's speech, pretending it's a phone call and answering by saying, "Yes, Commissioner?"
Now you look like an important big shot who can't wait for phone calls, knows the commissioner, and might be Batman.
Renting a tuxedo seems like the way to go, but don't do it! Everyone knows that tuxedos are rentable. A tuxedo fools no one. Instead, buy clothes at a fancy store and keep the tags on. Then return 'em later!
This can be tricky because you have to avoid spilling all night, but you kinda want that extra pressure to keep your from spilling. Nobody's going to believe you're a wealthy Google Brain Surgeon if you've spilled shrimp cocktail down your front five minutes into the reunion.
Well, well, well, someone sure got fancy since high school! Are you an international business mogel? A professional soccer player? An heir to the royal throne, perhaps?
For extra believability memorize some british-isms like "Smashing!" or "Tally ho!" or "Tut tut, mustn't question my accent, that would be improper."
A common high school reunion tactic is to pretend to be married, and then describe your favorite pillow as if it were your spouse. "Oh my husband? He's so supportive, a great listener, he has a very important job and he is really good at it." You might think you've really pulled one over on your former classmates and proved to them that you are, in fact, lovable. But this is a pitfall!
If you're "married" how are you gonna date and eventually marry the person you've loved since tenth grade? Or hook up with a popular kid and finally put to rest all your insecurities? Or seduce and then transmit an STD to your old bully?
Don't worry, it's not insider trading if you don't know how the stock market works!
Consider saying something like, "Buy aluminum, trust me, my subordinates and I came across some info while in my New York City business-mansion." (Your British accent will really sell this one, but remember to say "aluminium" instead of "aluminum".)
They didn't invite you to any parties back in high school? Well, make them wish they had by showing them how fun you can be and how hard you can drink! If this gets out of hand and you end up puking, aim for someone who was mean to you.
And honestly, depending on how many people wronged you back then, this might be the only tactic you need. Try to eat something really gross beforehand. Something milky.