Song Zhou didn't follow Jiang Jingchen in because he went to look for evidence.
The principal was also asked over by Song Zhou. The three of them had a fight and Yuan Jiaojiao was pushing all the blame to Tang Beibei, saying that it was her fault. That would not do, so he had to find an excuse to shut Yuan Jiaojiao up.
The evidence that Song Zhou found was a surveillance footage from the corridor where they fought. The surveillance footage would undoubtedly show everyone what happened and that Yuan Jiaojiao was the one who made the first move.
Now that she had lost the fight, she cried to the teacher and asked Tang Beibei for apology and compensation? This was extortion!
The principal scolded Yuan Jiaojiao with a cold face. "Don't you know your mistake? You were the one who started things both times, Yuan Jiaojiao!"