偏心，感性不理智，他这种人只是按照自己的喜好来对待学生而不管这个学生是否优秀。

偏心，厚此薄彼

这位老师很偏心。

A wise teacher never shows favouritism.
聪明的老师从不显露偏心。
The teacher shows partiality to him.
老师对他偏心。

你所描述的内容是这位老师对待学生的原则问题，并非性格。可以说这位老师为人师表却品德败坏、待人偏袒、颠倒是非。

描述性格时可以说性格内向（少言寡语）or性格活泼开朗等。

I don't understand what's going on here in this discussion and I don't understand how CAN you understand all these crazy drunk chinese sticks dancing around here. I just wanted to post a comment on a chinese discussion. :D