A Marine platoon sergeant is the senior noncommissioned officer in the unit, which generally is composed of about 40 Marines. In most cases, a platoon sergeant holds the rank of staff sergeant, pay grade E-6, and has been a Marine for at least four years, at least two of them as a sergeant. Many Marine platoon sergeants have taken special small unit leadership training, especially the Infantry Unit Leadersâ Course at Camp Geiger, North Carolina.