I take it you guys are as good as I am at drawing hard and fast, inviolate borders between things.
It just seems unnecessary. Our beginner class is geared towards white/yellow belts. But higher ranks will come in all the time for work on basics, to work on their teaching skills, or just to get some extra cardio.
Our intermediate class is geared towards geup ranks from green (7th geup) on up. But it's not uncommon for yellow belts to come. They may get a bit lost, and we let them know that we're not slowing things down to their level, but they still learn. Black belts are in this class all the time.
Our advanced class is geared towards upper geup ranks (say 3rd geup or higher) and black belts. But, again, it's not uncommon to have lower ranks attend, and they certainly benefit from the class.
If we were a big commercial school, it would likely be necessary to be more rigid. But for us, this works just fine.