Gender: girl
While serching for C. Erster, I came across three Czarna Ersters, numbers 5, 6, and 7. I believe that C. Erster #6 and #7 are the same person due to their identical names and similar addresses. C. Erster #7 has the address ALEXHOF 41 5 while C. Erster #6 has two addresses: REIMER 21 1 and ALEXHOF 41 5. Keeping in mind that these two entries may be the same person, we can see that perhaps C. Erster moved from the first address to the second on perhaps the date given in the "Notes" section for Czarna Erster # 6.
I have added Erster, Chana Hinda as a posibility. Her birth date makes her an appropriate age for a high shcool student at the time the card was signed.