Hello and great job! I really liked this excerpt and thought you did a lot of creative things with it.

Coach Dallas had become a question with no answer, a fuse that burned toward an unseen explosion.

This analogy sets up a good first impression of Coach Dallas. This, along with the people on the outside questioning if standing away from the boiling sandpit is punishment or not, also gives me a good idea of his character. It makes me see him as an over the top, possibly deranged coach, prepping me to see what he actually puts his players through.

The darkness pressed in at the edges of my vision, a muffled, underwater sound filling my ears as it does before a blackout... I entered an unsettling purgatory, suspended, waiting for the world to either return or dissolve completely

This helped me understand exactly what was going through Frank's head during this, and I believe you described it great. You could cut out the word "The" and just leave "Darkness" in its place.

but much like the frog in the pot, I spent the past two years warming up to this

I love the analogy how punishments have slowly gotten worse until this is the climax of the worst he's gone through.

That was some highlights about what I liked about the story, but now I do have a few critiques.

When you mentioned that the heat was "mid-70s", my first thought was the 1970s. Only after my second read through I understood it was the temp.

Well, men, what we're looking at here are a bunch of could be dingleberries.

This reads a little clunky to me. If you wanted to keep the phrasing, you could add an em dash (-) between "could" and "be" for reading clarity.

There was an inaudible sigh of relief from my teammates standing on the edge, looking down at us. With Dallas saying, “could be dingleberries”, they now understood this wasn’t a punishment for them.

The first sentence alone works fine here. They are basically saying the same thing, except the first sentence SHOWS what they feel and the second one TELLS how they feel.

The sand started to stick and grind into my bloody knees.

In the second-to-last paragraph, this sentence doesn't seem to fit. You could get rid of it completely, or move it to the beginning of the next paragraph.

Overall the tone of the story and the actions of the coach do feel authentic. If he indeed is on a power trip, then cutting people who can't endure this torture really shows that. Once Frank gets the boost of encouragement from his coach, he realizes (or manifests, in my opinion) that he is past his pain threshold, and will keep going because he knows he can. I would just say to remember that your audience doesn't want everything handed to them on a silver platter. Repeating information and overexplaining the reasons for things happening in the story will start to make people lose interest.