My Teacher, The Stranger is the thirtieth chapter of the Oushitsu Kyoushi Heine manga.
Ernst tells Heine that he finally knows who he really was, and asks that they have a chat in Prince Eins' castle. Heine refuses, but Ernst forcibly carries him to his carriage, where they then head back to the palace instead. He reveals that he's Maximilian's cousin, from whom he often heard about Heine and the princes. Heine asks if he knew Ralf von Fuchs, but Ernst denies knowing him, though he says that he knows one Adam von Fuchs. He also denies any involvement in the princes' recent troubles, but Heine is still suspicious.
They arrive at the palace, where Leonhard is waiting at the entrance for Heine to praise his homework. Licht is there also, and before Ernst leaves, he reveals in front of everyone that Heine used to teach at a church for poor children before he came to the palace, but there's no more information about that. Licht is surprised that even an adult with connections could not find out much on Heine's past, but Leonhard is put off by the fact that Heine had other students before them. For the rest of the day, Leonhard sneakily follows Heine around, and when Heine confronts him, tells him that he no longer wants him to be his teacher, since he's not important to him. He then runs away, but makes it obvious that he wants to say something to Heine. Heine tells him to write it down, and Leonhard asks if his former students are more important to him than him, since he volunteered to teach them. Heine tells him that all of his students are precious to him, especially him, since he's never met a student like him before, who hates studying so much. However, he's glad to have met him. Leonhard, flattered, gives Heine his homework, which turns out to be full of mistakes, so the next day he had to relearn it again.