I'm missing something then. I buy the extended girls 9.5 for my daughter who wears a 10.5-11 in regular j's and they fit. A regular 7.5 grade school is equal to a 7.5 mens
GS sizes have always been equivalent to mens sizes. When you were young squatch, you went from GS 6.5Y to a mens 7 as your foot grew. There was no conversion when you crossed over.
I personally wear a 9, 9.5 in jordans and have 3 pairs of the GS extended shoes which are also a 9Y, 9.5Y.
Only women's shoes get the size conversion you mentioned.