[Nyororin]

Whoa... As everyone else said, get your passport NOW!!!
And don`t send the application - expedited or not. Go to the office personally, and have it made while you wait. The chances of getting it in time are extremely low otherwise. Especially now. Summer is coming up, and a mailed application could take 3 months. I am simply in shock that you don`t already have one.

I would suggest bringing more than one "dress up" outfit. My guess is that you`ll need those FAR more than "get dirty" clothing. Japan has a tendency to take clothing very seriously, so unless you already dress very nicely to begin with... Chances are your normal clothes are going to look too casual in almost any situation. T-shirts and shorts that were totally acceptable for wear in the US suddenly became suitable only for pajamas and in-house wear once I came to Japan.
Expect to have to give several speeches in front of the students of the school you`ll be attending (greeting, saying thank you, plus whatever events they will want you to take part in.) so unless they have a uniform lined up for you, you may also want to have a suit or something approximate to one.

You will not need the sunglasses. No, wait, you may feel you need them, but your host family will probably be shocked if you wear them and chances are they will be banned at your school. Sunglasses are a no no in Japan.

It isn`t hard to find things you need in Japan, so I wouldn`t be too worried about small items. You`re also going to probably want to pick up notebooks, pens, pencils etc once you`re here as those available in Japan always seem so much nicer than those in the US.

Oh, and I would toss in another large folding bag. Chances are, you`ll be taking a lot more home than you brought and it`s expensive to buy another bag once you`re here.