She was discovered to be disguised as a boy and then bullied... Asai Kokoro SDMUA-035

To help her bullied younger brother, older sister Xinqing disguises herself as a boy and infiltrates his school. She discovers the bully's weakness and successfully stops the bullying, but conversely, her greatest weakness is revealed: she's a woman. Resistance is futile; pleasure descends… she becomes a beautiful girl turned into a male urinal. Until the painful and sorrowful ending that everyone covers their eyes…