Japan visa rules help!!?

I just got back from a 1 month exchange in Japan, now I want to go back in September to stay for 6 months, but I do not have time for the whole 2 month visa/certificate of eligibility process. Japan rules say that you can stay up to 90 days without a visa. Is it possible to go for 90 days, come back for a week, and then go back for another 90 days without getting a visa/certificate of eligibility?? By the way I am 16 years old if that helps any. Thanks for any help!