If you don't bring your own, you'll be given a pair of guest slippers by the school. These tend to be either too big or too small and they seem to be designed to fly off in directions you have no intention of going. Bring shoes that have a rubber sole that won't make black marks on the floor. Rain Gear Be prepared for the rainy season in August. You should bring a waterproof jacket or raincoat, especially if you're only means of transportation will be a bicycle.
Umbrellas are available in abundance here and many are very inexpensive.
Otherwise you may have to pay duty in Japan on boxes containing items totaling over a certain monetary amount. Check with the post office in your home country. Your boxes should arrive right when you need them if you send them in July. To give you and idea of how cold it gets here in Iwate, the most that I've ever worn at one time was lined wool pants, wool tights, silk long underwear, a couple sweaters, and a thick down-filled coat. Of course, I'm in the northern part of Iwate and your area might be slightly warmer or you might be used to snow country.