House parties are always a complicated thing. You never want to be too dressy like you're going to a concert... BUT you don't want to be too casual like you're watching movies in your basement on a rainy Saturday. So you have to find that balance. I think you can easily get away with a tshirt and jeans, but it's all about what you pair with it par example...
Avengers shirt: Macys
Holler for the "BOY VERSION OF ACTION ITEM"
Resolution Shirt: Action Item (Resolution Tour)
Batman Snapback: Hot Topic
Marilyn Top: Wet Seal
Plaid Blazer: JcPenney
Angel WIng Earrings: Hot Topic
Necklace: Anthony Li
Now this wasn't a house party, it was a mall show. However I think the same rule applies.