Lejb Arie Aryech Buchner was bursa student in Dom Sierot. Bursa student means an apprentice teacher (1924-27) at the orphanage while he was a student in Warsaw. He was studying at Państwowe Seminarium dla Nauczycieli Religii Mojżeszowej [State Training College for Teachers of Mosaic Religion in Warsaw]. Korczak was actually teaching at that college.
I do not know how and where Buchner survived the Holocaust. He ended, however, in Israel and there he did a great job by translating to Hebrew numerous of Korczaks books.
Please help me with more info about him.
Below, You can easily locate him on these two photographs as he is the only teacher there.