I skipped my tenth, as none of us were fully independent of one another-that high school thing still happens then. I am highly suggesting that you DO break down and wear a tuxedo and perhaps try and get A bow and cummerbund in your school colors. My hs was orange and black so it was veery easy. An Orange bow tie and cummerbund with the black tux and guess what? I won for best dressed. So, buck the trend to go casual and dress up instead. You may just impress your fellow class members more than you know! BTW-that reunion for me was the 30th one and I'm not sure in light of Internet and Facebook and all if a 40th will happen, but if it does, it'll be formal for me all the way!
Jeans with a suit coat :D
Sorry, I'm from Oregon, where that is a completely legitimate way to dress formal and casual all at the same time... It's even better if you have a plaid patterned, or other work style shirt to go with.