Onegai Teacher

Kusanagi Kei, a high-school student living with his aunt and uncle, has an encounter with a female alien. This alien is revealed to be a new teacher at his school. Later, he is forced to marry this alien to preserve her secrets. From there, various romantically-inclined problems crop up repeatedly.

(Source: Anime News Network)

Note:

-The title was never "- sensei," the Japanese characters phonetically spell "teacher".

-The chapter count includes one extra chapter.