I think that all Maryland teams with a "U" designation are middle schoolers only. HS teams are by graduation year just like LI.

However, my understanding is...

U15AA is mostly new 2018s who reclassified from 2017 at the end of their 1st time attending 8th grade

U15A is actual 2018s from birth paired with 2019s who are too old for U13 (U15A approximates U14)