Because no one was willing to step forward to identify Zhang Lei, even though everyone felt he might really have done something wrong! But without evidence, there's no truth, so the school couldn't do anything to him...
Moreover, the school definitely had to consider the impact this matter would have on the school itself, so the final action taken against him was quite low-key.
It was just that his position as class teacher for first class was removed, and under strong protest from all students in first class, the class he taught was swapped with another math teacher.
And the person they swapped in was Cao Yong!
