Have them "Bop" the downbeats. Maybe some students play the first and third sixteenth notes (down-up eighth notes).

Tell them they are providing a valuable service of helping the sixteenth note players stay on track.

You could also have the entire section(s) do this and add one student at a time with the printed sixteenth note runs. This exercise will clearly demonstrate who can adequately play the runs, and who are contributing "vagueness" to the line. This might work best in sectionals since it could take more time out of your full ensemble rehearsal.