Student Visa and other complications

This may be a rather complicated question, but here it goes:
I wish to study economics at a Japanese University starting April 2009. I'll graduate from High School this June. Rather than stay in my home country doing nothing, I'd rather take this chance to move to Japan starting in July and have some fun while I wait for April.
The issue is this, though: I read student visas can only be obtained outside Japan and with the sponsorship of a Japanese institution (in this case my university). Since it'll be more than 6 months till classes start (and 4 months before I take the EJU), I doubt I'll get the student visa.
So, is it possible for me to live in Japan from July to October (when the admission period at college starts) on a tourist visa and then, once I talk to my univeristy, change it to a student visa?
Assuming my 90 days run out, if I go to, say, South Korea and come back, do I get another 90 days? How long do I need to be away?
Also, if I can't change my visa into a student visa while in Japan, do I have to go back to my home country? if so, for how long?

If you haven't got a Certificate of Eligibility (COE) to be a student, then I guess your visa question will be a separate matter altogether from your intention to study in Japan.

Also, which country are you from? Visa conditions will depend on your nationality.
If you're from a 3rd world country, you will need a sponsor or CoE before you are able to enter Japan. Many 1st world countries have arrangements in Japan ranging from 14 to 90 days.
I've never been in the situation you describe, so I can't give you a better answer :(
For more details, try googling japan mofa visa (mofa stands for ministry of foreign affairs).