Everybody's spot on with the contacts, hairgel, and outfit helping him look better, but don't forget that he's also been working out (probably for the first time in his life) because of Anjou. His clothes are normally kind of baggy/ill-fitting so it's hard for classmates to notice, but with a tailored costume from Inuyama, combined with everything else, Seto is making a huge visual change.
GET IT, MY DUDE!!