1A system in which the members of a police force (or other emergency service in which 24-hour cover is needed) are divided into platoons which replace one another on duty as a body.
2(a) US. A system in which pupils are split into groups and undertake separate classes or activities, especially (in earlier use) in rotation, so that a longer school day might include periods of exercise, vocational work, etc. (now historical). (b) South African a system in which two sets of pupils and teachers use the same school for half a day each; (also occasionally) a system in which pupils use classrooms in rotation, being taught outside for the rest of the time.
3Baseball American Football
A system in which two or more players or groups of players replace one another on the field of play as circumstances dictate.
Late 19th century; earliest use found in The New York Times. From platoon + system.