Awww. IMO-- sessions suck! If you were going to do sessions, I'd do a faster group, and a slower group-- that's it. another option would be to have one group have the track from say... 10-1, and have the other group go from 1-4. After 4, it's an open track.
Another route, (I've done a bit of road racing) would be to steal from the pavement track day system. They have a flag displayed at the start/finish at all times. A green flag means: "open track/no restrictions", a yellow flag means: "The track is getting crowded, and/or there are beginners on the track. Exercise caution, and passing is allowed on the outside only". A black flag means that it is "Expert/fast novice practice only", a blue flag means that it's practice for the beginners/slow novices, and a red flag means that the track is closed due to something slippery on the track, injury, or track maintainance. At many tracks, the beginners wear an orange vest over their leathers. This indicates that the rider is new, so observe caution in their presence.
These are some possibilities, but I prefer my dad's way of prevention. Before I did anything that could be potentially dangerous, my dad used to tell me-- "Don't let anything stupid happen!". I guess that sums it up! --L*64