Mr. Clown,
Don't know how you do the on-field session(s) - with real teams scrimmaging? Or, with participants simulating teams? I've never been a fan of having real teams scrimmaging, because you can't control what 'looks' they are going to throw at you, and you can't 'stop' things to discuss issues. Simulated teams works much better.
I am always surprised at how many folks really don't understand keys, and how to stay with them during the play (or come off of them). Lots of them can 'tell' you their keys, per the manual, but they have no idea how that translates onto the field. So very many just forget their key at the snap and become ball/ball carrier watchers.
One of the most beneficial on-field sessions that can be done is identification and observation of keys. Using a group of sideline/deep guys, have the participants simulate eligible receivers, with just two or three DBs. When the 'team' moves to their positions, have the 'crew' (H, L, B, or H, L, S, F, B) point and shout out their key(s). Then run a play, and have a someone (A or B) commit an exaggerated foul as the play develops, and see if the appropriate official picks it up. Run 'reps' of three plays per 'crew,' then rotate. Keep this up until everybody has had three or more 'reps.' Start very simple, and, as the drill progresses, have the receivers shift (after initial keys have been IDd), forcing the crew to re-identify keys. Run simple pass patterns at first, then patterns that cause the crew to stay with their keys as they run across the field, etc.
It is a very satisfying feeling to watch guys flounder in their first rep, then, by the end of the session, they all nail their keys, and the right folks are looking at the right things at the right times. Makes me jump and shout and scream "Yeah!" when they get it right, and you can see they "get it."
Similar can be done with the R & U. Keys don't change as often, but they do change occasionally. But, they need practice staying with their keys the appropriate amount of time, and coming off their primary key at the right time, to look at a greater threat, etc.
Try it and see what you think. The trick is to keep things moving, so they don't get bored waiting their turn. If there are enough participants, and enough field space, break into multiple groups, so they get more reps in the time available.
Robert