I just created a Google Code SVN repository for storing my school projects and homework, and to allow easy transferring between school and home.
Its default directories i
There is no clear answer to this question as it depends on what suits your projects best.
From a maintainability point of view I would almost always prefer the second way; but if your projects get bigger and might prolong, it would be better to use separate repositories per project.
Besides that: Please check if you really need Google Code services for your homework because its purpose is to support OSS. You can always use SVN locally or even through SSH, so you could also put your repositories on an USB stick or some computer that can be accessed remotely; you don't really need hosting for that. There could also be privacy concerns.