All it takes is to be skinny and spend a summer running high volume. I ran sub 17 in my first race after doing that. A school that has nobody sub 18 has to be either small or incredibly unlucky, i.e. nobody wants to do it.
"Talent" is an expression of ignorance. There's a reason for all athletic ability whether people know it or not. But sub 17 is not even in that realm of mystery. Sub 13, maybe.