Hana Yori Dango

Makino Tsukushi is the only poor student at Eitoku Gakuen, a school for the ridiculously rich and privileged. Eitoku Gakuen is ruled by the Flower 4, otherwise known as the F4, a group of four boys who come from extremely powerful families: Domyoji Tsukasa, the leader and heir of the Domyoji World Finance Group; Hanazawa Rui, the introverted son of a large company; Nishikado Sojiro, a player who is the heir of a tea ceremony school; and Mimasaka Akira, a madam killer with ties to the underground. If a student gets on the F4's bad side, he/she gets a red notice and is bullied and driven out of school.
Makino hopes to pass her days quietly - without drawing any attention to herself - but one day, she stands up to Domyoji in defense of her friend. The next day, Makino receives the dreaded red notice. Even though she is harassed, Makino decides to keep going to school and declares war right back on the F4 because she is a "tough weed." Her resolve gets the attention of her crush, Hanazawa Rui, and oddly enough, rouses romantic feelings in her worst enemy, Domyoji Tsukasa.
(Source: DramaWiki)
~~ Adapted from the manga “Hana Yori Dango” (花より男子) by Kamio Yoko (神尾葉子).