Best Lens for Sports Photography

Honestly, you are getting a lot of answers from folks who either never ever shot high school sports, or if they did have completely forgotten what's involved.

Presumably, you are going to end up shooting a variety of sports, not just one basketball tournament. Even for basketball (which involves much more than tight action shots if you're planning to do the assignment correctly and provide proper views of the entire scene), you need a wide variety of perspectives, and the ability to swing into those opportunities quickly.

Anything other than a zoom will be a huge compromise. (I'm primarily a prime shooter, so don't bother to flame me about this suggestion). You already have the best AF-C body ever made, so the choices of zoom lenses that will track well are fairly broad. If you aren't already experienced with shooting heavy lenses extensively (such as the 50-140), then you don't really have enough time to learn how to do so proficiently. I think your best bet is the 70-300, or possibly the 55-200 if you don't expect to be doing much longer shooting (football and soccer especially require longer focal lengths). Down the line, the 50-140 would be best for basketball in a close H.S. environment, but you would want far more time to learn how to properly shoot it.

Feel free to shoot at high ISO to use proper shutter speeds (1/500 minimum, but 1/800 would be better for high school sports). Image noise isn't a concern for action shots, so ISO 6400 isn't a problem. You don't need a fast aperture to get the shot, but great shots will be better with that wider lens - down the line. However, getting several fine shots is a whole lot better than getting no great or even good shots with a demanding new lens you haven't yet mastered.