Well, I most certainly have trained beginners to sling with stones from the get-go, but I always hit the deck or crouch behind a tree.
The thing to consider with this situation though, is that they woke up at 3 o clock in the morning, and in order to compete they had to run in these rubber sandal kits they made themselves, so their feet were bloody from running up and down Texas' finest jagged hills and canyons. Then they swam in a frigid river for a few miles. After 8 hours of that, they participated in the slinging challenge, exhausted and very delirious, trust me on that one.
(They were carrying cactus pads in their hands without removing the spines!
)
Different story... a few years ago I was teaching a friendly gentleman and two 12 year old's how to sling. They had each bartered for a sling from me, and I wanted them to be able to leave that day with a relative understanding of how to sling safely. The problem was that there was only ONE tennis ball, each had to wait his turn.
One of the boys wandered off, I didn't think much of it at the time, and thought he could teach himself how to sling. He picked up the ugliest jagged stone this side of the atmosphere, put it in the sling, and began doing helicopter the way most beginners do, rigid, tense, and very fast.
From what I gather, he drew his arm out as if doing jumping jacks, didn't release, and the stone skated across his scalp like a plow through a field.
I heard a yell, went to see what happened and found him touching the top of his head, blood trickling down. Thankfully, there was a retired special forces medic around and he fixed him in short order.