Here is a very effective way to proceed:
Review the necessary academic material for your group's level (road-map, keys, solo sections, etc.)
When sight singing, the following system allows for maximum director feedback in a minimal amount of time:
their parts with dynamics and articulations.
their parts on La or Da (Solfege if they have the training)
their parts (Say only what you play)
this allows the director to discriminate
between which section is doing what and quickly correct potential mistakes. If everyone sizzles, then this becomes a much more difficult task. There can be a few variations, however most of the groups receiving superior ratings use some form of this system. BEST PRACTICE.
Video is coming soon on this one.