An Academy lineup is a lineup where all the 11 players in the initial lineup are youth pulls from your club who, on average, are no older than 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. If one of the players is substituted due to an injury during the match and a player enters the field who is not a youth pull, the points will be rewarded nevertheless.