I'm a teacher in XX University

因为大学是一个大的团体机构， 你只是其中的一份子， 用in

I'm a teacher of the Class I

班级是一个小单位， 而非你莫属， 用of.

填空

In less then two weeks,I will graduate from the university.

In less then two weeks,I will (graduate) from the university.

leave or graduate