By a Twist of Fate, I'm Attending the Royal Academy in Disguise

Cesia, a mere servant, is attending the Royal Academy in place of the lazy, pampered daughter of the viscount she works for, all for a massive monetary reward upon her successful graduation that would allow her to live the rest of her life without any worries. She spends her days avoiding the feeble bullying of the ladies of the Academy while being worked to the bone at the viscount’s mansion—until just before graduation, when she gets tricked and her pay completely vanishes! Furious, she vows to get even, no matter what it takes, when out of nowhere, the second prince shows up and offers to help her? A spunky heroine with determined resistance as her motto and a mischievous prince who puts his life on the line for his country meet in this love story where anything could happen!

(Source: Cross Infinite World)