That's because he knows so well his ex greatest ability, manipulation. The teacher also a little bit of in fault here as she just plainly believed the explanation from Daidouji without any kind of verification which in turn causing Yukishiro being cornered by the entire class except her own friends. Should Ichinose told the truth about who is responsible for the bullying, that ex would be able to pointing on Ichinose instead since she can placate someone and then push them into insanity easily while the teacher at the same time also a little bit gullible.
