Crap. This exact thing happened in the fourth quarter of a heated contest, JV boys, just tonight.
In the heat of the moment (pressure cooker, back-and-forth game), I gave a whistle and issued a warning for delay of game (their first) to be recorded by the keeper of the scorebook. That got their attention! They hustled very quickly from the bench area to take their proper spaces in the bottom two spots.
Then, in the bleachers while observing the varsity game, I noted the casebook ruling that a warning for delay of game was not warranted, but that I should have, as others have stated above, just told them to get into their positions. Shucks, they'd have done that before any T was warranted.
This just shows the importance of studying what's posted on this discussion board. Had I more than glanced over the thread earlier today, I wouldn't have half-kicked it.
Turned out okay, but now I know what's right for next time.