If your flash is dictating the exposure, then to a large extent shutter speed becomes irrelevant to stopping the motion. Try something like ISO 800, 1/250 f/2.8 with a little FEC one way or the other, and see how it does. This assumes regular ambient exposure would be typical HS stadium cavelike conditions, i.e., in the ISO 3200 1/400 f/2.8 range or worse.
If the ambient is better than that you may not need the flash at all except perhaps for fill. But most fields are not evenly lit at all, and the ends are really dark. I usually find I need flash in the endzones, and maybe in the red zone, but can get away without it towards midfield. So I usually shoot the 70-200 on one body with flash, and the 400 on another without it.
Nill
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