With many scores available as free downloads, you can give them a list of scores they can study. The Faure solos are good examples of what can be done with simple means, also the Saint-Saens Fantaisie, for starters. You can also give them a list of good parts and bad parts. But, being young composers, I don’t know if orchestra needs to be the focus. I am working on this material, and it is time-consuming. I think showing how we correct things is excellent, but that is too detailed for what you are doing. Other than basics, like chords with too many notes.
The errors I am seeing most these days in younger composers is, due to computers, overlapping notes between hands, re-playing notes too soon, and impossible reaches.