Because at that time, while her family wasn't as obviously superior as now, the teachers were simply biased towards those cleaner and neater children with better families. She didn't have any nice clothes and always wore old ones. She was always going in and out and didn't have time to stop and tidy herself up. Thus, the teachers also were not fond of her. Her classmates didn't like students that the teachers didn't like too.
Thus, at that time, she was always subjected to bullying.
But these were no big deal anymore. She later grew up and when she met her old classmates again, she would simply laugh it off when they brought these things up.
Yu Minmin instructed Yu Chengcheng, "Study hard. Don't bother with those things that aren't important. I'm going back."
"Don't worry, Sis. The school treats me like a precious gem now. I'm not as tragic as you were. You were even bullied while you were in school. Nobody dares to bully me now."