keep in
phr. v..phr. v.. to make someone stay inside a place, usually as a punishment or for safety.
phr. v.. to detain someone within a building or room; frequently used in educational contexts regarding detention or in medical contexts regarding patient confinement.
The teacher kept the students in after school.
Because of the heavy rain, the coach decided to keep the players in for a theory session.
The hospital staff decided to keep the patient in for observation to ensure no further complications arose from the surgery.
usually takes a person as the object; often implies a lack of choice for the person being kept.
contrast with 'keep out' (preventing entry); in a school context, 'keep in' is the standard term for the act of giving a student detention.
The teacher kept in the students.The teacher kept the students in.while technically possible, when the object is a person, it almost always goes between 'keep' and 'in'.