This is "The Story of Queens and School Girls for One Thousand Years"....
Juliet, a princess, was in love with the main character. But she took over the throne, and needed to work for the people, holding back her love toward him. One day, she finally crossed the line, and slept with him without knowing the impending threat from the neighboring countries.... One thousand years later, they all meet again at a high school, as a student or as a teacher or as an older sister. The main character lives a peaceful life.
But triggered by a meeting with a transfer student, his life changes dramatically.
"I've been looking for you." The curtain on the battle for the main character raises....

[From a forum post by VInsanity]