Pika la,
Personally, I recommend that you look around more schools, since you're planning to come this summer. (Make sure you visit the schools early in July rather than August, because you won't be able to see much student activity at that time.) High schools in Japan are varied in academic standing and specialization, and you should choose one that best fits your needs.
I don't know the school shown in the link. It does appear to be conveniently located in Ikebukuro. It was originally a commercial school (for students who expect to take on jobs after high school) and has expanded into "regular" high school curriculum. I don't see any arrangement for students without Japanese language capability, so I have a feeling you should check further into that. I also have a sneaking suspicion that a school of this background has rigid school rules--I'm just guessing, so you must look into that, too.
There should be many more schools available for you to check. Also, I recommend you not to stick to the Toshima-ku area. Many high school students commute on trains for quite long distances to the schools of their choice, rather than going to schools close to home. You'll get used to commuting very easily, and this way you get a broader range of choice.
I still recommend Tokyo Metropolitan Kokusai High School, which is located close to Shibuya. It is about a 20-minute ride on train. The school is relaxed and free, with school rules not overly rigid, but still with a good academic curriculum. If you have your parents' friend as guardian and is able to get a long-term visa, I think you should give this school a try. Your parents can write the school in English for inquries here.
http://www.kokusai-h.metro.tokyo.jp/indexe.htm
Note: This school accepts student in both April and September. Foreign students are required to prove that they had a valid, long-term visa, certificate of graduation from junior high school (or equivalent) and write an essay in Japanese or English. The quota for foreign nationals is small, so you better hurry!!
Another alternative in Toshima-ku is Rikkyo High School, a high school affiliated to Rikkyo University, founded by an Episcopalian and known alternatively as St. Paul's. Christian schools tend to be freer and relaxed than most other private schools. Because of its international background, they very likely have arrangements for special Japanese language study for you. You should inquire with them also.
http://www.rikkyo.ne.jp/grp/ike/
Another school that came up to mind is Myojo Gakuen in Kichijoji. An American acquaintance of mine and his Japanese wife had their child enrolled in this school, although the boy eventually went on to international school and then to college in America. This is also about 20 or 30 minutes away from Ikebukuro. This is a very laid-back, free school.
http://www.myojogakuen.ed.jp/high/high_index.html
Once again, look around more and don't be so concerned about sticking to Toshima-ku. Have your parents' friend make phone inquiries into schools like these. Because there are many, many schools in Tokyo, you should look for a school that is best suited to what you plan to do in Japan and what direction you plan to take in the future.
As for overlapping/repeating grades because of the transfer, you may have to accept that, since you don't have the Japanese language capability of Japanese high school students and were taught a different curriculum. Maybe the adjustments will work in your favor (moving you up to your appropriate grade) when you return to your home country (Canada?).
Good luck!
