An Academy lineup is a lineup where all the 11 players in the initial lineup originate youth pulls from your club (or were part of the team when you got it) and have an average age of below 18. An academy lineup is only complete if there are 11 players on the field when the game starts.
A manager often plays this lineup in friendlies to get achievement points for it. The amount of points awarded is 35. It must be noted that one will only receive the points when the average age is below 18. If one or more of the players are substituted during the match and the players entering the field are not youth pulls, the points will be rewarded nevertheless.