Maybe because she was a loner? I hate to think that they would do... all that to her though. They went WAY too far...

Before it was probably because of her appearance/social status, but since everyone wanted to date Tohma, and they were jealous of Misao for dating him.

Honestly, I think before the whole confrontation scene with Yashino and Misao it was because she was an outcast and seemed strange to everyone else. But afterwards, when Tohma and Misao started dating, it was just that. Girls were jealous and angry with her because they thought Tohma deserved something better, I guess? Especially with Yashino, it's revealed that she most likely crushed on Tohma as well and that's why she did what she did to Misao. Even then, she took it like WAY too far.

If I saw someone get bullied for no reason, I would stand up for him/her

I would too, but in Japan, where the game took place, I guess it's different there, people just turn a blind eye towards bullying sometimes because it will inconvenience them.

Since Yoshino was the leader of the bullies, they all probably followed her lead. Yoshino picks on people weaker than her, so that's most likely why she targeted Misao at first. When she found out Misao liked Tohma, she was obviously jelly so she probably told all her bully friends to help her make Misao's life as miserable as possible.