I was doing soccer at around 1/500th and it was fine. Depends on what part of the action you are shooting... running, sprinting, blocking, tackles...each moment has a peak where it is somewhat slower and you can get away with a slower shutter... take a QB dropping back for a pass, he puts his arm back for a pass...it hits a peak going back right before it goes forward..
It also depends on how the action is oriented toward the camera...on shots coming right AT the camera, or straight away from it, slower shutter speeds will stop motion adequately well...but action that is moving at right angles to the camera will require higher shutter speeds...a running back in the backfield and taking a handoff and running down the line and right toward the camera position can be "Stopped" with a moderate speed...a guy streaking right down the sideline with the camera up close to the action will require a much,much faster shutter speed for an adequately stopped-looking picture. If the games are at night, when most varsity games are, you probably will be stuck somewhere between 1/180 and 1/500 at f/2.8 at ISO 3200 at many HS fields. Often, the field will have poor illumination evenness as well...often mid-field to the 20 is decently-lighted, but the goal line and end zone areas are like The Abyss.