Tomboy in traditionalist society
I have a solid remembrance of the plot but need the title and/or author.
Setting: world of 1850-60s technology (railway, but no electricity, aircraft, cars & trucks) after apocalypse eons ago (my assumption, nuclear war)
Plot: Elia Kenzi (or similar spelling), a tomboy (my est. 14 to 16 years old), living on Emerald Isle, an island (size of Japan, my est.) isolated from the rest of the world, returns home from an all-boy residential school after being outed as a girl (girls are not educated, not even in reading & writing). She runs away from home again to study at Open City, the only city on the island open to off-islanders, when she befriends 2 humanoids. Later, she's contacted by her best friend who tells her that Elia's brother has been declared missing after not returning from the Test, a rite of passage for 15- to 17-year-old boys. She decides to enter the Test as a boy to rescue her brother.
The book was written in the 80's or early 90's. Book's cover had a teen-age girl wearing a backpack and walking up a hill.