Subsequently to the JOI Ninja Contest last year(http://codeforces.com/blog/entry/5262 ), JOI (Japanese Olympiad in Informatics) will hold a contest mainly for high-school students who enter IOI this year(of course, everyone can participate)
the site is here : http://cms.ioi-jp.org/
I hope it will help you prepare for IOI :)
UPD: The ranking system is not a perfect one,it is refreshed with delay now. In the actual contest, it will be refreshed once in 5 or 10 minutes,but some trouble may occur,so please don't trust the ranking absolutely.