There's this manga book that I've read but forgotten.

It's a manga with school, cross dressing and fantasy elements, and I think the illustrations were manhwa styled.

The graphics are similar to a manga called Girl Queen.

In the background history of the story, she was a child of a prestigious family and was locked in a temple, after she had lost her memories after a tragic accident where she summoned something (a dragon?) and it killed the sister of her guy friend.

Random Note: This was when she had long hair. Later she cuts it off.

She got fed up with being isolated in the temple and decides to run away. She gets lost because she has no money and is saved by a guy, who was actually her childhood friend from before the accident, although she doesn't remember him. Later on, she joins a school for the elite, not knowing that her mother once attended the same school. In a fencing match to prove herself, she becomes best friends with her opponent, a girl with long black hair, who is actually the guy who saved her dressing up as a girl.

Turns out he has to dress that way because he's impersonating his sister, under the orders of his mother, who runs the school. She was an ambitious woman who wanted her daughter to be the best, so when she died, the mother forced her son to transform (or was it the use of magic to change gender) into his sister. His mother and the main girl's mother used to be best friends up until the accident where the main girl accidentally killed the "sister", causing a rift between the already unstable rivalry/friendly relationship between the two mothers.

And then the sister's spirit comes back and possesses another student at the school to get revenge...

And that's all I remember, since it's been over 5 years. The details might also be off.

Please help me.
And no, it's not Tiara.