I've found no evidence of metaphorical use of "school miss." Even the term "school miss" to mean "schoolgirl" is unfamiliar to me. I've found a few instances of the term on Google, from British fiction of a couple of centuries ago, or from current romance novels set in the Regency period (early 19th century) in England.
"” As I found the family congenial besides the young men, two daughters, one a school miss of fifteen, the other a girl of eight or nine...
"” "Wounded in the mind? You sound like a school miss with the vapours. Why not speak plainly? Your wits were disordered.". "I was ill, sir. Like one with a fever."
"” She said nothing, for she felt unaccountably shy. She, who had had half the gilded youth of England at her feet, found no light bantering word with which to meet this man; and beneath his ardent gaze she felt herself blushing like a school miss at her first ball.
"” "I love him, Ellen.". "Love! You talk like a school miss instead of a middle-aged woman. He doesn't love you. He wants a housekeeper and a governess.
"” The family plots an elaborate deception when Sir Philip brings home the vulgar and greedy Mrs. Budge, while Lady Carruthers passes off her daughter as a school miss and sets her own cap for the young Earl of Denby--whom the daughter also pursues.
As far as I know, the term is not used in contemporary English, but if any of our members has heard or read it in modern English, please let us know.