It's about an ex-yankee girl who turns sweet to get the guy she likes (cliche I know). The guy is in charge of morals for the school but is secretly the next head of a gang. She was known as "the red devil" during her yankee days. Any help is appreciated.

Pretty sure it's Kyuukyoku Kareshi ni Yamitsuki Kanojou, from the one shot collection Megane - Hazushitemo Ii Desu ka? (4th story)
You can also find it in Ojou-sama no Himitsu as an extra story

That's it, thank you!