Cpl Kimberley Hey is said to have asked the teens if they thought they were hard — then hit any who raised their hand.
She also told under-18s who got questions wrong to raise their arms and loosen their belt before punching their stomach.
One lad cried after she randomly hit him and others during six months of training at the Army Foundation College in Harrogate, North Yorks.
Signaller Hannah Harwood said: “If she was in a bad mood, she would punch us. She had no business behaving in this way.”
Cpl Hey, 34, of 3rd Regiment Royal Logistic Corps, called it banter.
She denies eight counts of battery.