Just got back from Hokkaido and went to the Hokkaido Japanese Language School in Sapporo (http://www.hokkaido-jals.com), the school was very good. I did two weeks of class, you can start any Monday and they pair you with people at your level (after an online assessment). There is an enrollment fee and then you pay weekly. Teachers are very good, classes are up to 8 people and follow Minna no Nihongo (2 chapters per week). Classes are held in the morning, 3 x 50 minutes. There is lots of info on the website. The school has a very good location in the centre of Sapporo and they have a nice lounge/cafe space for studying in the afternoon. I will definitely return there next year. June to August is very busy with their summer course, but I think September should be less busy and so easier to get a spot (I booked about a month before and they had limited space, but I was put in a group that was exactly my level so it was perfect). The school had lots of people from the US, France, Thailand, South Korea and Scandinavian country.
Sapporo is also a really cool city, excellent food specialties (soup curry, shio ramen, fresh seafood, yummy sweet treats, tons of micro-breweries for awesome beers), cheaper than Tokyo/Kyoto. I stayed at a guesthouse called Yuyu Guesthouse, it was really good and I walked passed the fish market every day on my way to school (about 15 minutes walk), so sometimes grabbed a fresh seafood donburi for breakfast, amazing memories. If you’re looking for board, Yuyu and their sister guesthouse Waya Guesthouse accept people on working visa for cleaning in exchange for bed and meals (that might not be compatible with class schedules though).