While my initial inclination is to agree that slacks and a sportscoat would be fine, my second reaction is that there are going to be at least a dozen 13 year-old boys there, most of whom will be wearing a suit. I think if 13 year-olds will put on a suit, I myself will wear a suit. It doesn't have to be an interview outfit, but I would go with a suit.
Why do the homunculi get offended, lash out, and call everybody fat? Do the you realize that there are tons of people that have 6-packs that wear larger sizes due to things such as muscles and/or bones; or are you just an angry, little man?