assuming they can do them, and are just not getting the cues, you can try a few of these
to start in the right position, take a good plank form
-middle finger should be pointed forward and force should be spread throughout the entire hand, not just fingers or heel or whatever.
-inner elbow joints should be facing in toward each other
-scapulae should be slid slid down the back, not hunched up near the neck
-upper back should be rounded enough to pull the sides out. I've had people use the cue to (ok, this gets goofy) imagine your armpit hair being pulled outward away from you, and move accordingly.
-stomach/abs should be braced, as if they're about to get a gut punch. not sucked in, just braced
-but should be slightly tucked, as part of that brace
-glutes should be activated, not clenched, straightening the hips, almost trying to pull the legs back, in order to make them in line with the torso
-quads somewhat flexed to straighten the leg
All of that should make a good plank. A lot of the perfectly strong dancers at school had the problem of the sagging in the back, and those cues helped them a lot. *shrug*
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