Jeans are still considered by most (adults) as "casual wear" and are definitely not appropriate for "semi-formal". And if your teen or young college, be aware that most girls your age will prefer the "adult" definition because that's how they intend to dress. She's wearing a hot cocktail dress (semi- formal) and you're only wearing jeans? Kinda makes you look like a kid. Just saying'. Semi-formal is midway between "dressy casual" which is nice shirt/jeans/jacket/tie-or-no-tie, and "formal" which is tie/fancy-shirt/tux. So, "semi-formal" is a good shirt (color is your choice), tie, nice regular suit (matching nice pants and jacket). The kind of suit you would wear as a groomsman or guest in a semi-formal wedding (formal would be tux) or at a serious management job interview. Now, since you are a kid (I think) it might be more relaxed (black jeans) but do you really want to be the only guy who shows up wearing them? If you are taking a date to this thing, you could always ask for her preference, too. I'd really suggest a good dark-colored suit, and maybe a really cool colored shirt (pumpkin, maroon, green or whatever goes best with the suit color) with a great tie. And if you have the funds, a cool vest (maybe with a pattern, but that coordinates with suit) would be good with it, too.