Yep, agree with this. YY/YY is the least confusing representation, and users will know how to parse this.If you name it by grad year, then you'll have parents who think their older kid can play in a younger division. "But he's in 8th grade...." No one working the signup table wants to deal with that ten times every hour.
My guess is we refer to them by the pair of years, same as we did 10 years ago. "14/15" means kids born the second half of 2014 or the first half of 2015.
I know "red shirted" kids who are a year behind in school from the start, even before rarer (but possible) grade movements. Additionally, the cutoff point doesn't 100% align with school enrollment cutoffs points anyway. School year would be far more confusing.