Different levels of "unblocked" access exist depending on whether you can modify your system settings or if you are restricted to the browser.
Chrome OS supports Android apps. If your school hasn't disabled the Google Play Store, you can download the official mobile version of the game. osu unblocked chromebook
If you are using a personal Chromebook or have administrator access, you can install the full PC version of osu! via Linux. Different levels of "unblocked" access exist depending on