Some unblocked game portals host flash/html5 rhythm games similar to mania. Search for:

A high-performance version of the game hosted via TurboWarp , which runs Scratch projects at 120 FPS—ideal for the fast reflexes needed in mania. How to Play Without Installing Software

To install osu! Mania on Chromebook using Linux:

Sites like Interstellar or other "browser-within-a-browser" tools can bypass local filters to reach rhythm game sites.