fall in
phr. v..phr. v.. to move into a line or a specific position, usually when following orders in the military.
phr. v.. to take one's place in a military formation or organized line; an intransitive command or action indicating readiness and alignment.
The captain shouted at the soldiers to fall in.
As soon as the whistle blew, the recruits scrambled to fall in for the morning inspection.
The sergeant major watched with a critical eye as the exhausted infantrymen struggled to fall in before the general arrived.
primarily used in military or highly disciplined contexts as an intransitive verb.
contrast with 'fall out' (to leave the formation); it is helpful to explain that 'in' refers to the internal structure of the line or group.
The teacher told the kids to fall in the classroom.The teacher told the kids to line up.'fall in' is specifically for formal formations; 'line up' is the standard term for schools or casual groups.