I can't remember the name of two shoujo mangas. One is a oneshot about revenge and the other is a full length school life manga:

ONESHOT MANGA
The main girl finds out her boyfriend is cheating on her and he dumps her for the other girl. She tells her male best friend that she wants revenge, and he creates an elaborate plot to make it so that the ex-boyfriend's new girlfriend finds out he was two timing and the new gf dumps him.

In the end the female MC feels bad and helps he ex-bf get back with the new gf and the female MC ends up with her male best friend.

FULL LENGTH MANGA
Also a shoujo school-setting manga. In it the female MC becomes friends with a really tough, violent girl ( I think she has a nickname like she-wolf or something ). There are also two male MCs. The female MC has a crush on one of the guys and tries to get him to fall in love with her. The romance is a big part of the story but the main storyline is the friendship between the female MC and the tough girl.

Towards the end of the manga the female mc gets kidnapped and calls out for the tough female best friend, rather than her boyfriend and she saves her.