I'm the Villainess, So I'm Taming the Final Boss, Vol. 2 (light novel) - Mai Murasaki, Sarasa Nagase

Cross-Dress for Success! When news of demon attacks in the Duchy of Mirchetta spreads across the kingdom, Claude finds the prospect of potential rebellion somewhat bothersome, but it’s nothing he can’t handle as the new crown prince. Aileen, however, recognizes these events as the start of the sequel game that could end with her fiancé’s doom! To protect him in secret, she concocts a plan to infiltrate Misha Academy disguised as a boy so she can get to the bottom of things. But she’ll need to move fast or risk Claude discovering her ruse. That’d be as good as game over...!