any dramas about student-teacher affair?

Secret Love Affair - male student, female teacher Maybe not the ending you want. I suggest you read the reviews first =)

My Rainy Days :) 

Koukou Kyoushi 1993 and 2003 versions
Majo no Joukan

I just know these.

Unstoppable High Kick - It's not the main focus (but hey, it's a sitcom: nothing is!) and it mostly ends up being an onesided crush (an adorable one!)... and the ending *spoiler*is left open, although he finally does act on feelings properly*. But regardless, it's like the best sitcom ever. Truly a must watch.

Koukou Kyoushi 1993 and 2003, Majo no Jouken, Koishite Akuma

Kinkyori Renai (2014)
Japanese movie with Yamashita Tomohisa.
Kururugi Yuni is a smart high school student. She excels in all subjects except for English. A new English teacher begins work at Yuni's high school. His name is Sakurai Haruka and he is very handsome. Yuni begins to receive one-on-one tutoring from Mr. Sakurai Haruka. Soon, Yuni develops feelings for her teacher and she realizes that she is now in love. Yuni then confesses to her teacher. ~~ Based on the manga series with the same name by Mikimoto Rin.

I too recommend Kinkyori Renai!!! It's a movie but loved it!!!
http://www.gooddrama.to/japanese-movie/close-range-love-movie ...you can watch it from there if you like!!

Kinkyori Renai is pretty good too